The Elderly Novice Virtual Organist

The Pipe Organ in a Nutshell

This page is a beginner's guide to the pipe organ. It should enable the reader to develop a good understanding of the largest, most complex, and most expensive of all musical instruments.

It was originally developed for my benefit, as I knew next to nothing about the organ a few years ago. Although I am clearly no expert, everything has been carefully researched from multiple sources. Where these are inconsistent, I have been able to take a fresh and critical approach to resolve them.

0 Introduction

The pipe organ is a musical instrument that produces sound by driving pressurized air called wind through pipes of various sizes and types. Pipes giving the required notes are selected through keyboards played by the hands and feet. The console also has controls to enable and facilitate selection of pipes of specific types with different tonal qualities.

A large pipe organ has thousands of pipes, which can produce a wide variety of sounds covering the full range of human hearing. However, each pipe is voiced to work at a specific wind pressure, and (unlike an orchestral instrument) has a fixed pitch, volume level, and sound quality (timbre). In other words, a pipe is either ON or OFF, depending on whether its airway is open or closed.

As the sound made by each pipe is fixed, volume and sound quality are set by selecting appropriate combinations of pipes. This art, called registration, is unique to the organist, and is a fundamental part of the interpretation of organ music. The quietest sounds of the organ are produced by using only a single soft pipe; the loudest by deploying numerous pipes simultaneously (which on a large organ could be several hundred).

The pipe organ is perhaps the most complex device built by man before the telephone exchange. Each is built individually according to user requirements and the building in which it is to be used; there is no mass production. Each pipe is unique, and voiced in its location to achieve a proper balance with the others. This goes some way to explaining why pipe organs are so expensive; the cost of a large instrument is measured in millions of dollars.

Pipe organs vary considerably in size, age, style, nationality, and numerous other design factors. The smallest organs have only a few tens of small pipes; the largest has over 33,000 (some of which are enormous). There are organs over 500 years old that are still being played. The pipe organ is not only the largest, most complex, and most expensive musical instrument; it is also the most diverse.

Pipe organs are often thought of as being used primarily to accompany congregrational singing, being found in churches and cathedrals. However, they are used for a wide variety of music, and are also installed in concert halls as well as other public buildings, and even in some homes. The organ repertoire spans over 500 years, and includes a large volume of secular music as well as sacred. Few would dispute that the supreme composer for the organ was Johann Sebastian Bach, who wrote rich contrapuntal music for it that continues to both delight and challenge organists.

The pipe organ is essentially a complex machine with limited expressive capabilities. But despite this, it has inspired considerable respect, and has been described by Mozart and others as "the king of instruments".

1 Console

Consoles reflect the general diversity of the pipe organ, with many different styles and little overall standardization.

The main controls on the console can be divided into the following areas:

Keyboards: 1 to 7 Manuals, with (on most organs) a Pedalboard.
Stop Controls: Typically located to the left and right of the manuals, sometimes above.
Thumb Pistons: Located under the keys of the manuals.
Kneeboard: A vertical panel located behind the pedalboard.

Keyboards and stop controls operate the Stop and Key Actions that select which pipes are to sound; they are present on all organs. Thumb pistons and controls on the kneeboard are mainly playing aids or used for expression; these may not be present, especially on older instruments.

The organist sits over the pedalboard on a wooden bench, whose slippery surface permits the pivoting necessary to reach all the pedals with both feet. The front edge of the bench is usually slightly convex (with a radius of about 15m, or 50 feet) to assist in this. To cater for organists of different physical stature, its height is normally adjustable.

In organs with mechanical action, the console must be positioned close to the pipes, and often forms part of the organ case (when it may be called a keydesk). With electric action, the console and pipes may be in completely different locations. See Mechanical, Pneumatic, and Electric Actions.

1.1 Keyboards

On the organ, there are two types of keyboard, with different names to distinguish them. Manuals are played by the hands, the Pedalboard (or pedals) with the feet. On this page, the word keyboard is used to cover both types.

Each keyboard (including the pedalboard) normally operates on a separate Division of the organ. Manuals and their corresponding divisions are often identified by Roman numerals (I, II, III, ...) as well as names, and the pedalboard by the letter P.

Organ keyboards have the same layout as piano keyboards, with the 12-note octave comprising five raised keys in two groups (called sharps) and seven lower keys (called naturals). The lowest note in both manuals and pedals is always C, which at a given Stop Pitch are at the same frequency; however, stops in the pedal division are typically at lower pitches than those in manual divisions.

The keys of the manuals and pedals operate the key action; this and the stop action determine which pipes will sound. As keys open and close airways, each note is sustained at a fixed volume for as long as the key is held down, and terminates immediately when the key is released. This makes organ keyboard technique quite different from that of the piano, where a wide range of touch dynamics is available, and each note decays after the key is struck - see Comparison with the Piano.

1.1.1 Manuals

A manual is similar to a piano keyboard, but normally covers no more than 5 complete octaves (61 notes), compared to the 7 octaves or more of the piano. In historical organs and many modern ones, there may be fewer keys (56 or 58 is common). Nonetheless, with stops of different pitches, the compass is potentially greater than that of the piano. At standard 8' pitch, the middle C on the manual produces the same note as middle C on the piano.

Overall, the keys of an organ manual are similar to those of the piano, and should present few difficulties to pianists. However, organ keys are much shorter, and very slightly narrower (less than 1%) than piano keys. The action of the organ requires less weight to produce sounds, except with mechanical key action, which in some old instruments is very stiff and heavy.

The sharps are most commonly black, with white naturals (as on a piano). However, on some organ manuals, the naturals are black, with the sharps a light color (as in the above photo); this is simply to give a better appearance. But it is important for playability to distinguish sharps and naturals by the use of contrasting colors.

The majority of pipe organs have from 2 to 5 manuals. Having multiple manuals provides separation of divisions, with quick and easy changes from one division to another. Each hand may play on a separate manual; for example, to provide contrast between solo voice and accompaniment. Some pieces are physically impossible to play on a single manual, due to collisions that would occur between the hands.

But having a large number of manuals makes ergonomics more difficult, as each additional manual is higher up and set further back, making it more difficult to reach. Although the lowest three manuals are normally horizontal, manuals above these may be angled toward the player to ease access.

1.1.2 Pedalboard

The compass of the pedalboard is about half that of a manual; normally two and a half octaves, or either 30 or 32 notes. Although pedalboards on historical and electronic organs often have fewer, 30 pedals are required even for some baroque music; however, little music of any period requires 32 pedals. The pedalboard is significantly wider than a normal manual, as spacing of pedals is over two and a half times that of keys in the manuals.

A pedalboard may be either flat or concave. In a concave design, the outer pedals are higher than those in the middle, making them easier to reach. Most modern pipe organ pedalboards are concave, as this is generally preferred; however, low-cost pedalboards, or those of historical instruments may be flat.

A pedalboard may also be either parallel (rectangular) or radial (radiating). In a radial design, the pedals are further apart by the kneeboard than by the bench, providing a more natural angle for the feet; the downside is the variable spacing between the pedals. Radial designs (AGO standard) are more common in the US and the UK, whereas parallel designs (BDO standard) are usual in continental Europe.

Sharps in the pedals are much more elevated than those in the manuals. Although they are often also black, the use of contrasting colors here is unnecessary, and wood finish may be used throughout.

While sharps must be played with the toes, the naturals may be played with either toes or the heel. However, early pedalboards did not support use of the heel; although the organ works of J S Bach contain many elaborate and fast-moving pedal lines, all can be played using only the toes. But later works require use of the heel. Shoes are of a special flexible type with leather soles, but some organists play in socks.

1.2 Stop Controls

Most of these are for speaking stops that open and shut the airways to complete ranks of pipes, thus enabling the sounds of the organ to be selected - see Pipe Organization.

Apart from speaking stops, in this area may be found accessory stops; these are:

Couplers: Combine divisions of the organ.
Tremulants: Create a wavering effect by varying the wind supply.
Toy Stops: Sound effects (such as drums) and semi-musical effects (such as bells) that do not involve pipes.

The traditional type of stop control is the drawknob, which is pulled out to select the stop, and pushed in to deselect it (hence the phrase "pull out all the stops"). Commonly-used types of control that require electric stop action are the tilting tablet, and the similar rocker tablet; the former is hinged on the end, the latter rocks on a central axle. An organ may feature more than one type of control; for example speaking stops may use drawknobs, with couplers selected by rocker tablets. Stop controls are most commonly located on jambs to the left and right of the manuals, but may also be found in rows above the manuals (particularly tablets).

Stop controls are grouped into the divisions they form part of. Beyond that there is no standardized form of layout, which is dependent on the nationality and age of the organ, among other factors. While flues and reeds are normally separated, reeds usually appear first in British and American organs, but flues first in European organs. In British and American organs the lower-pitched stops usually appear last, while in European organs they appear first. The names of reed stops are often engraved in red.

1.3 Thumb Pistons

These are playing aids that make it possible to select or deselect a large number of stops in a single operation. Before their introduction, each stop had to be operated individually from the stop controls. This limited the extent to which the registration could be changed while playing, even with an assistant (called a registrant).

Some pistons may have fixed combinations set by the organ builder (mainly in older instruments). But most pistons are user-programmable in one or more of the following three ways (from oldest to newest):

Setter Board: This has on/off switches for each stop for each piston.
Hold and Set: The piston to be set is held down while stops are selected.
Setter Piston
: First the required combination is selected, then this piston held down while the piston to be set is pressed.

The majority of user-programmable pistons are programmed with a combination of stops that can be selected by simply pressing the piston. This causes all the stop controls involved to be physically moved as if they were operated manually; there is thus visual indication of the effect, and stop controls and pistons can be used interchangeably. This mechanism is known as a combination action.

A piston may be either general or divisional. Generals are programmed with stops for all divisions, and when operated affect the entire organ. Divisionals are programmed only for a division, and when operated leave stops in all other divisions unaffected. There is usually a General Cancel piston to cancel any drawn stops, thus facilitating a fresh selection.

Some pistons are reversible; that is, the first push activates a function, and the second push de-activates it. Reversible pistons are used for quick access to accessory stops such as couplers and tremulants, and for special reversible operations such as tutti (full organ) and sostenuto (sustains notes after the keys have been released). Reversible pistons do not usually move the stop controls; instead an indicator light shows when the function is activated.

An important function is the combination stepper, which allows the organist to step through a sequence of general combinations. This is pre-programmed with all the combinations required for the music to be played, in the order in which they appear. Only two pistons are required (typically duplicated on each manual and kneeboard for quick access); one to select the next combination, and the other for the previous one. This is increasingly being used in preference to managing large numbers of divisional and general pistons.

Mechanical thumb pistons were developed during the second half of the 19th century; however, setter functions that could capture a combination were only practical with solid-state circuitry. But with computer electronics, it is easy to provide a large number of memories and various means of using them. Many new organs allow numerous sets of presets to be stored; for example, to cater for different organists. MIDI makes it possible to record a performance in terms of stop, key, and expression pedal activity.

1.4 Kneeboard

This vertical panel behind the pedalboard (also called kickboard) may contain the following types of control:

Expression Pedals
Toe Pistons
Coupler Controls

1.4.1 Expression Pedals

These enable the volume to be changed continuously while playing. Previously keyed and held notes are affected, not just those played after the pedal is operated. However, they do not allow adjustment of the sound produced by each pipe, which remains fixed. And they are not true volume controls, as they also change the sound quality.

They are of two types:

Swell: A swell pedal exists for each Enclosed Division, to gradually open and close the shutters of the swell box.
Crescendo: The crescendo pedal successively adds and removes stops in a preset sequence.

Expression pedals did not exist during the baroque period. The swell pedal is a standard feature of romantic-period and later organs, except for those that are specifically neo-baroque. The crescendo pedal was introduced in the mid-19th century, and is found mainly on the larger organs.

1.4.2 Toe Pistons

These are essentially equivalent to the Thumb Pistons on the manuals. Some may be divisional for the pedals, others duplicate some of the general pistons on the manuals, so they can be activated when the hands are busy. Also called toe studs.

1.4.3 Coupler Controls

These pedals are found on organs with mechanical key action. They are "hooked down", and returned to the up (off) position by means of a spring. They are used to activate Couplers, and may also be used for Tremulants and the Barker lever.

2 Pipe Organization

A Rank is a set of pipes (one for each note on the keyboard) that produce the same type of sound.
A Stop is one of the sounds that may be selected by the organist. Most correspond to a single rank, but some stops are multi-rank.
Stops are grouped into Divisions, each of which has its own character and (usually) keyboard.
Divisions may be combined using Couplers, which may also be used for other functions.

2.1 Rank

This is a row of pipes producing the same type of sound. Each rank normally contains a separate pipe for each note on its corresponding keyboard; thus a manual rank typically requires 61 pipes, a pedal rank 30 or 32 pipes. However, sometimes a rank covers only part of the keyboard compass (in particular, the lowest notes may not be present). In other cases, a rank may be extended above and/or below to support unification or octave coupling.

Pipes within each rank vary greatly in size; for example, for a 61-key manual (5 complete octaves), the speaking length of a flue pipe for the lowest note is 32 times (2⁵) greater than that for the highest note; see Pipe Lengths and Pitches.

Although all pipes within a rank are normally of the same type, in practice a completely consistent sound across the range is not achievable; both quantity and quality of sound may vary noticeably from the lower end to the upper. In some cases ranks are hybrid, using different types of pipe for the lower and upper notes.

A large organ may have well over one hundred ranks (the most is 464); the smallest possible organ has only a single rank.

2.2 Stop

The term stop properly refers to the valve that opens and closes the airway to a set of pipes that forms one of the sounds of the organ. However, it is commonly used to refer to any of:

the control on the console used to operate the stop mechanism
the set of pipes operated by the stop mechanism (one or more Ranks)
the sound produced by these pipes

The set of ranks controlled by a stop and its sound is sometimes referred to as a register (hence the term registration). In a quality classical organ there are usually more ranks than registers, as some stops such as Mixtures involve more than one rank, and each rank is used in only one stop. A basic organ specification usually contains the number of stops; for example, 51 / III+P means that the organ has 51 speaking stops over three manuals plus pedals.

But on some organs, the same rank is operated by more than one stop control. In these cases, there may be many more speaking stop controls than there are ranks, and the number of stops based on these controls is then meaningless (the only meaningful criterion is the number of ranks). The following sections describe four such types of stop; all except the last are compromises.

2.2.1 Unified Stop

Unification is the creation of multiple stops at different pitches from a single rank, thus providing a greater stop selection for a given number of pipes. It is not available with mechanical key action, and requires rank extension to cover the additional notes. It is used extensively in Theatre Organs, where there is limited space for pipes, and to augment the function of some other types of organ with fewer ranks than would be considered ideal.

For example, instead of making three stops at 16', 8', and 4' pitches from three specific ranks (requiring 183 pipes on a 61-key manual), the same stops can be made from a single 8' rank. Extending the rank an octave above and below requires an additional 24 pipes, giving a total of 85 pipes; there is thus a saving of 98 pipes.

However, this re-use of pipes results in some limitations. Instead of each rank being scaled and voiced appropriately for its pitch, the same scaling and voicing are used for all unified stops. In particular, higher-pitched stops intended to reinforce harmonics are inappropriately at the same strength as those at base pitch. Furthermore, unified mutations are tuned according to the organ temperament, not pure as required (see Special Stops). Only one pipe will sound where notes coincide, not multiple pipes. Apart from these issues, unification generally results in a more homogeneous and less complex sound.

2.2.2 Borrowed Stop

The term borrowing is sometimes used in a similar way to unification, but is used here in its original sense to describe re-use of the same rank at the same pitch in different Divisions. This predates unification, and being at unison pitch requires no rank extension.

For example, Principal 8' will often appear in both Great and Choir divisions, but would normally be separate ranks voiced differently to be compatible with that division (that in the Great would be stronger than that in the Choir). With borrowing, both divisions would share the same rank. Perhaps more commonly, a 16' manual rank may be borrowed for the pedal division, as this rank would be more expensive to provide.

However, use of the same rank in more than one division means that it is not likely to be ideally balanced for either division. And again, if the same note is played in both divisions, only one pipe will sound, not two.

Borrowing is used in some quality classical organs, but should always be indicated. Typically, the division from which a borrowed stop is taken is given in parentheses. For example, if a 16' trumpet in the Great division is borrowed for the pedal division, the stop name in the pedals might appear as "Trumpet 16' (Gt)". There are also small and cheaply-built church organs that use borrowed stops under different names.

2.2.3 Combination Stop

Combination stops are made up from multiple ranks that are used elsewhere, as could be programmed on presets. They are another technique used on the aforesaid economy organs to make few ranks look like many. The issues with unintended multiple use of the same rank and pipe here are obvious.

2.2.4 Divided Stop

Divided stops are found mainly in small historical organs, particularly from Spain. They allow a stop to be applied independently to the lower and upper parts of the manual via two separate stop controls. This enables the left and right hands to play with different registrations, even when there is only one manual.

Unlike the other uses of the same rank by more than one stop control, this is not a compromise. The rank is split between the two stop controls, which thus form a single logical stop.

2.3 Division

Stops are grouped into divisions, each of which is normally associated with a keyboard. Thus an organ with three manuals and pedals normally has four divisions. Each division has its own name and characteristic, and can be thought of as a separate instrument. Pipes within a division are usually physically grouped together to give a cohesive sound.

Division names and characteristics depend on the country of origin, age, and other stylistic aspects of the organ. A British or American organ with three manuals might have divisions named Great (usually played from the middle manual), Choir (usually played from the lower manual), Swell (usually played from the upper manual), and the Pedals. The Great organ is meant to sound broad and majestic, the Choir will have a smaller and plainer sound, and the Swell division would be Enclosed.

Although divisions are conceptually independent, they may be combined using Couplers, causing notes played on one keyboard to also be applied to one or more others.

2.3.1 Enclosed Division

Most organs, other than small instruments or those of pure baroque style, have at least one enclosed division. This often has the name Swell or the national language equivalent (for example, in French this is Récit). The pipes in such a division are enclosed in a chamber known as a swell box. This has shutters (similar to a Venetian blind) that may be opened and closed using a swell pedal (one of two types of expression pedal). The swell box was introduced in London at the beginning of the 18th century, and does not feature in baroque or earlier music.

The swell pedal is not a pure volume control, as opening and closing the shutters changes not only the quantity of sound, but also its quality. Placing pipes within a swell box compromises the sound; therefore, classical organs generally have at least one unenclosed division. A division may be only partly enclosed; that is, have some of its ranks within the swell box (and so under expression), but others outside.

2.3.2 Floating Division

On some large organs, there are more divisions than there are keyboards. In some cases, a keyboard may normally operate simultaneously on more than one division. However, there may be one or more divisions not normally connected to any keyboard; such a division is known as a floating division, and may be connected as required by means of a coupler.

2.4 Coupler

The basic purpose of a coupler is to combine selected stops from more than one division, so they can all be played from one keyboard. This is equivalent to connecting keyboards so that they operate in tandem. These are inter-division couplers that operate at unison pitch.

Many organs also have octave couplers that couple an octave higher or lower, rather than at unison pitch. These are primarily intra-division couplers that couple an octave higher or lower on the same keyboard. There may also be couplers that are both inter-division and octave; these couple to another keyboard at octave pitch.

Coupling normally results in the coupled pipes being sounded in addition to those that would normally sound. However, each division may have a Unison Off option, so that only the coupled pipes sound. This facility is particularly useful in combination with super-octave and sub-octave couplers to transpose an octave. It may also be used to reassign manuals; for example, to play division III on manual II instead of manual III, one would use Unison Off on division II with the coupler III → II.

2.4.1 Unison Coupler

This basic function enables the stops selected on one division to be played from a keyboard corresponding to another division. For example, if a manual is coupled to the pedals, pressing a pedal has the effect of also pressing the corresponding key in that manual. This is common practice in music with an elaborate bass line (such as that of J S Bach), to give an incisive sound that could not be achieved using stops in the pedal division alone.

With mechanical coupling, the mechanical linkage results in the keys on the coupled keyboard being physically moved in tandem with those played. For example, if the Choir division is coupled to the Great, when the player presses a key on the Great manual, the same key in the Choir manual will also be depressed. There are also special latchable mechanical coupler controls on the Kneeboard. With electric coupling, there is no visible effect, but the end effect is the same as with physical connection. Electric coupler controls are normally located with the Stop Controls.

Organs typically provide a set of couplers enabling each manual to be coupled to one of a lower number, or to the pedals (it is not normally possible to couple the other way). For example, on an organ with three manuals and pedals, the basic set of couplers is: III → II, III → I, III → P, II → I, II → P, I → P (6 couplers). These can be combined as required. Then (for example), if couplers III → I and II → I are selected, pressing a key on manual I will have the effect of also pressing that key on both of manuals II and III. All these couplers are inter-division; that is, they couple from one division to another.

Some organs have bass couplers that couple the lowest note played to the pedals. This enables the pedal division to be used by those not competent in playing the pedals. There are also melody couplers that similarly couple only the highest note played, to emphasize the melodic line.

A few organs have partial couplers that operate on only part of a division. For example, it may be possible to couple only the reeds from one division to another, leaving all other stops uncoupled.

2.4.2 Octave Coupler

On many organs with electric key action, it is also possible to couple at a different pitch, normally an octave higher or lower. Such couplers are known as super-octave or sub-octave couplers, respectively. This will result in keys on the linked division an octave higher or lower being sounded, in addition to the unison keys (unless unison off is active).

These are primarily intra-division, coupling the division to itself at a different pitch. There may also be inter-division super/sub-octave couplers, and thus a large number of coupler controls to cover the numerous possible combinations (which again may be combined). Super-octave couplers are often indicated with the number 4 (one octave above standard 8' pitch), and sub-octave couplers with the number 16 (one octave below). Similarly, unison couplers may be indicated with the number 8.

3 Pipe Classification

Drawing of a selection of flue and reed pipes

Stop names and types demonstrate the diversity of the pipe organ. There are many hundreds of stop names in several languages, which vary according to the country of origin and also historically. Those mentioned on this page are a somewhat arbitrary selection.

Many types of stop are referred to by several different names, often with only small differences in spelling. In some cases these may be confused; for example, Dulciana is a flue and Dulzian a reed, but Dulcian may mean either. In other cases, completely different names are essentially synonymous; for example, Gedeckt and Stopped Diapason.

The sound produced by stops of the same name also varies significantly from organ to organ, and also within an organ. Often the same stop name appears in different divisions of the same organ, but using separate ranks voiced differently to be compatible with each division. The most basic stop is Principal 8', which may appear in each manual division.

Nearly all pipes used in classical organs fall into two classes:

Flues: (labial stops) Produce sound by blowing wind through an aperture, in the same way as a flute.
Reeds: (lingual stops) Produce sound using a vibrating metal tongue, with amplification by a resonator.

Both function correctly only at a specific wind pressure; the sound from each pipe is fixed in pitch, timbre, and volume.

On a classical organ, the majority of stops blend with the others; they change or reinforce the sound without being heard distinctly in themselves. This type of stop is frequently referred to as a chorus stop, and is used in a manner analogous to mixing paint. However, some reed stops are intended to stand out from the others; such a stop is referred to as a solo stop.

3.1 Flues

These have no moving parts, and produce sound by blowing air through an aperture (in the same way as a flute or recorder). There is a foot through which air is blown, a body above that comes in various shapes and sizes, and in between a narrow hole. The air blown in causes the air column in the body to vibrate.

The pitch of the note depends on the length of the body containing the air column (the foot is not a speaking part of the pipe). The timbre depends on the shape of the pipe, and especially the scale (its diameter in relation to the speaking length).

Flue pipes may be either open at the top end or stopped; the latter are only half the length for a given pitch, but are able to produce less volume. Some pipes are semi-stopped, with an intermediate length/pitch ratio. They may be of either metal (normally with a circular cross section) or wood (usually with a square or rectangular cross section).

Good-quality metal pipes are normally made of a tin/lead alloy. These metals are expensive and dense, but are soft and easy to work; this allows the very fine adjustments necessary to achieve high-quality voicing. There is some dispute as to what effect the acoustic property of the material has on the sound, but organ builders agree that tin/lead alloy produces the best tone. Tin is the more expensive, but has a brighter appearance than lead; alloys higher in tin are therefore likely to be used for pipes in public view. Zinc and other metals are also used, but to save money or weight.

The majority (usually more than three quarters) of organ ranks are of flue pipes. They are divided into three categories according to their scale:

Flutes: Wide Scale
Principals: Medium Scale
Strings: Narrow Scale

These categories, together with Reeds give the four groups into which organ pipes are commonly divided. This is a convenient breakdown, as an organ typically has a similar number of stops in each group. They are often referred to as the four "tonal groups" or "sound families".

But there are gray areas between these three categories of flue pipe; for example, while Choralbass is usually described as a principal, some sources have it as a flute. Those of intermediate scale may be referred to as hybrid. Moreover, some flue pipes are conical, and may also be described as hybrid. These are often flute-scale at the lower end, tapering off to string-scale at the top. There are also inverted conical pipes that are wider at the top. And while flue pipes of different categories may sound quite similar, the sound from reeds varies considerably in both quality and quantity.

3.1.1 Principals

These produce the sound characteristic of a classical pipe organ (similar to an "ah"). They tend to be equally strong from low notes to high. Many such stops are named simply Principal; this is the most basic stop, and is usually at 8' in the manuals or 16' pitch in the pedals. Other commonly-used names for principal-scale stops include:

Choralbass: Strongly-voiced principal, usually at 4' pitch in the pedals. Used for cantus firmus.
Montre: French equivalent of Principal.
Octave: Principal at 4' pitch in the manuals, 8' in the pedals. Typically used with the standard principal.
Open Diapason: Used in British and American organs for Principal (Stopped Diapason is flute-scale).
Prestant: As Octave, but usually mounted at the front of the organ case.
Superoctave: Principal at 2' pitch in the manuals, 4' in the pedals. Typically used with the 8' or 16' principal.

3.1.2 Flutes

These produce sound that is softer and fuller-bodied than a principal (similar to an "oh" or "oo"). They tend to sound stronger in the high range, and weaker in the low range. Illustrative examples are:

Blockflöte: Wide-scaled conical or stopped flute, named after the recorder, with similar sound.
Bourdon: Stopped wooden 16' pitch flute of large scale, similar to Gedeckt.
Claribel: Open flute in the manuals, first made of wood, later of metal. Usually at 8' pitch.
Gedeckt: Very common stopped flute stop, usually of wood. Synonymous with Stopped Diapason.
Harmonic Flute: Open metal flute made to sound an octave above its length by means of a small hole at its midpoint.
Hohlflöte: Describes a variety of flute stops; may be metal or wooden, open or stopped.
Koppelflöte: Open metal flute of 8' or 4' pitch, that terminates in a short cone.
Nachthorn: Wide-scaled flute with a relatively small mouth. Produces a soft but penetrating sound.
Nason: Stopped wooden flute, like the Quintaton having a prominent twelfth.
Quintaton: Stopped flute of wood or metal, with a prominent twelfth.
Rohrflöte: Semi-stopped metal flute capped by a narrow open-ended tube (also Chimney Flute).
Spitzgedeckt: Wooden flute of pyramical shape with a dull tone having few harmonics. At 8' or 4' pitch.
Stopped Diapason: English equivalent of Gedeckt (Open Diapason is principal-scale).
Tibia Clausa: Stopped wooden flute of large scale, usually with a leathered tip.
Trichterflöte: Inverted conical flute, usually of metal, but may also be pyramidal of wood.
Waldflöte: Soft flute at 4' or 2' pitch, usually made of wood.

3.1.3 Strings

These produce sharper but weaker sound than a principal (similar to an "ee"). They tend to be stronger in the low range. They include those with names containing Viol or Geigen, and:

Dulciana: Metal string stop; usually the softest on the organ.
Fugara: Open metal or wood pipes, often tapered; tone has a sharp "stringy" quality.
Gamba: One of the earliest string stops, named after the baroque instrument.
Salicional: Nearly always of open cylindrical metal pipes; softer in tone than the Gamba.

3.1.4 Hybrids

All the following are conical, and range in scale from flute to string.

Erzähler: A light-toned and strongly tapered stop invented by E. M. Skinner.
Gemshorn: Conical flute stop tapering to string-scale at the top. Similar to the Spitzflöte.
Spitzflöte: Open flute of metal with pipes tapered to a point at the top, having a reedy or breathy sound.

3.1.5 Labial Reeds

These are flues designed to emulate reeds, often to imitate orchestral instruments. The sound tends to be mellower than that of a true reed.

Clarinet Flute: A flute stop, but with a reedy tone that may resemble that of the orchestral clarinet.
Echo Oboe: Similar to a soft Oboe; may also be a true reed.
Saxophone: A wooden flue, said to imitate the instrument better than the Brass Saxophone reed.

3.2 Reeds

Also known as beating reeds to distinguish them from free reeds, these produce sound using a vibrating metal tongue that is amplified by a resonator. Air is blown into a boot containing the tongue, which vibrates against a shallot, both held in place by a wooden wedge. A tuning wire enables the pitch to be adjusted by changing the vibrating length of the tongue. The resonator may be of metal or wood, but the boot is always of metal.

Reed pipes produce much more harmonic content than flue pipes. They vary greatly in quality, and include both the loudest in the organ and quiet ones. They tend to be strongest in the middle of the range.

The resonator reinforces Harmonics of the sound wave, and largely determines the volume and timbre. Longer resonators tend to give a stronger sound; unlike with flues, there is no fixed relationship between pipe length and pitch. Timbre is also affected by the thickness and curve of the tongue.

Resonators may be conical or cylindrical. Conical reeds include both strong-toned ones and imitative ones. Cylindrical reeds have prominent even-numbered harmonics; they include baroque reeds and imitative reeds.

Reeds may also be described as chorus or solo. Chorus reeds are normally used to reinforce the full organ. Solo reeds are intended to be heard as separate voices, often in quieter passages, and frequently imitate orchestral instruments.

Most reeds, like flues, are mounted vertically on top of the wind chest. However, some powerful trumpet-type solo reeds are mounted horizontally so that they protrude from the front of the organ case. These reeds, said to be mounted en chamade, produce loud sounds that may stand over full organ.

The examples that follow are categorized on resonator length.

3.2.1 Short Reeds

Regal is both a reed stop name, and a generic term for a family of reeds with short resonators.

Bärpfeife: Early reed of the Regal class, with a soft growling sound. The only examples are at 8' pitch.
Dulzian: Cylindrical baroque reed with tone similar to a bassoon.
Krummhorn: Cylindrical baroque solo reed with a short resonator that has a buzzing or bleating sound.
Musette: Imitates the instrument of that name (a small bagpipe); may also use free reeds.
Rankett: Regal-class reed, after the instrument (an early double reed with a muffled tone).
Regal: Has fractional-length resonators; produces a buzzy sound with little fundamental tone.
Trichterregal: Regal-class reed with funnel-shaped resonators.
Vox Humana: Regal-class reed, supposedly intended to imitate the human voice.

3.2.2 Medium and Long Reeds

Bombarde: Powerful conical chorus reed with a brassy timbre.
Clarinet: Imitative cylindrical solo reed, usually at 8' pitch.
Clarion: Trumpet-type conical chorus reed at 4' pitch.
Cor anglais: Imitative conical reed, synonymous with English Horn.
Cornopean: Chorus and solo reed like a large-scale Trumpet.
English Horn: Imitates the orchestral instrument (which is often called cor anglais).
Fagotto: Chorus reed of inverted conical construction. Not the same as Orchestral Bassoon.
French Horn: Imitative conical reed.
Oboe: Used as both a solo and a chorus reed. Larger scale than Orchestral Oboe. Also called Hautbois.
Ophicleide: High-pressure chorus reed with powerful sound, much like the Bombarde or Trombone.
Orchestral Oboe: Imitative conical reed at 8' pitch.
Schalmei: Baroque "color" reed. Usually cylindrical; some are conical. Also called Shawm.
Trombone: Chorus reed simulating the trombone; similar tone to Bombarde and Ophicleide.
Trompette en Chamade: Solo trumpet laid horizontally; can often be heard over full organ.
Trompette Militaire: Powerful solo reed of the trumpet family, with a brassy, penetrating tone.
Trumpet: Loud chorus reed stop, with inverted conical resonators. Also called Trompette.
Tuba: Large-scale, high-pressure, smooth solo reed. Not named after the orchestral tuba.

4 Pipe Lengths and Pitches

4.1 Stop Pitch

The pitch produced by a flue pipe depends on the speaking length of the pipe (that of the resonating column of air, which excludes the foot). Halving this length doubles the pitch, thus producing a note an octave higher. Doubling the length halves the pitch, giving a note an octave lower.

Although the pitch of a note is its frequency in Hz, the pitch of an organ stop is given in feet. This is the speaking length of the longest (and thus lowest pitched) open flue pipe, which produces the low C in both manuals and pedals. The use of feet to indicate pitch is convenient, as the length in feet associated with the note C is close to a power of 2. Each single-rank stop is usually marked with this number, which on a large organ may range from 1' to 32'.

For example, a stop marked as Principal 8' on a 61-key manual (five octaves) has pipes ranging in length from 8 feet to 3 inches (a factor of 32, or 2⁵). At this pitch, the lowest note on the manual or pedalboard is just over 64 Hz, and the middle C on the manual is at the same pitch (around 256 Hz) as the middle C on a piano. Octave 4' sounds one octave higher, Principal 16' sounds one octave lower, and so on.

The pipe length / pitch relationships indicated above hold only for open flue pipes. Stopped flue pipes sound an octave lower than open flues of the same length. There is no direct relationship between pipe length and pitch with reeds. Nonetheless, in all cases the pitch is indicated by the length in feet of an open flue. So, for example, the lowest note of a stop indicated as 16' will always have a pitch of about 32 Hz, regardless of the type of pipe.

A large organ can produce fundamental frequencies that span the entire range of human hearing, from 16 Hz (the lowest 32' note) to over 16 kHz (the highest 1' note). However, not all this range would be perceived as musical notes. Stop pitch is usually based on 8' in the manuals and 16' in the pedals; this is often referred to as unison pitch. Non-unison stops whose pitches are powers of 2 are said to be at octave pitch.

Stops of higher than unison pitch (4', 2', and 1' in the manuals) are used primarily to add brightness (harmonics), and are not usually heard distinctly in themselves. Many organs also have stops of 16' pitch in the manuals and/or 32' pitch in the pedals. These not only add depth, but are also heard as separate notes an octave lower. As these additional voices tend to clutter the sound, they should be used judiciously. For example, such stops are not normally appropriate in forms such as fugues, where the clarity of individual voices is vital.

4.2 Harmonics

A musical instrument (except percussion) produces notes consisting of a fundamental tone, plus overtones (harmonics) that are exact integer multiples of it. For example, when an oboe sounds concert A=440 Hz, in addition to the fundamental tone of 440 Hz, it produces a 2nd harmonic at 880 Hz, a 3rd at 1320 Hz, a 4th at 1760 Hz, and so on. This is referred to as the harmonic series, and the timbre of an instrument is determined by the proportions of its harmonics in this series.

A tuning fork produces a dull tone that is almost entirely fundamental, the flute (both instrument and organ stop) has a full sound containing a large proportion of fundamental, whereas a violin has a thinner and brighter sound much richer in upper harmonics (especially the 7th) with less fundamental. Harmonics that are odd multiples of the fundamental tend to give a harder/edgier sound than those that are even, but all harmonics add color.

All these harmonic components are fused by the ear into a single note, whose pitch is the fundamental frequency. In fact, given only the harmonics, the ear can often reconstruct the fundamental. For example, if an organ pedal note is played on loudspeakers incapable of reproducing the fundamental frequency (as will often be the case), the ear can nonetheless perceive the depth of the note if there is sufficient harmonic content; this would be the case with pedal reeds, but not necessarily flues.

Understanding harmonics is vital to the art of registration. While most reeds have a high harmonic content, flue pipes tend to be low in harmonics, and it is often necessary to give them more brilliance. For this reason, Principal 8' is often combined with Octave 4' and Superoctave 2' to reinforce the 2nd and 4th harmonics. This principal chorus may in turn be enriched with stops that reinforce other harmonics, including those that are not at octave pitch. These have base pipe lengths are not powers of 2, but fractions derived from the harmonic number.

The names of many of these stops refer to the interval in the musical scale, not the number in the harmonic series. For example, the name Terz denotes a third, but generates the 5th harmonic (which is E, the major third in the musical scale based on C). Similarly, Quint indicates a fifth, but generates the 3rd harmonic (G in the scale based on C). These are usually indicated as fractions in relation to the base pitch; for example, a Terz is indicated as 1 3/5' (= 8' / 5), and a Quint as 5 1/3' (= 16' / 3).

4.3 Special Stops

4.3.1 Mutation

Comprises a single non-octave rank, which may be either principal-scale or flute-scale. Mutations are present in nearly all organs, and are used only with unison/octave-pitch stops to reinforce their harmonics.

Principal-scale mutations include Terz (1 3/5') and Twelfth (2 2/3'). Flute-scale mutations include Nasard (2 2/3'), Terz (1 3/5'), Larigot (1 1/3'), Septième (1 1/7' or 2 2/7'), None (8/9'), and Quint (5 1/3' or 10 2/3'). For a full list and further information, see Mutations (Encyclopedia of Organ Stops).

4.3.2 Mixture

Comprises multiple ranks that typically include non-octaves (most commonly fifths, but thirds and occasionally other harmonic intervals may be included). Mixtures are present in nearly all organs, and are used only with other stops to reinforce high-frequency harmonics. They typically add a "bite" to the sound.

Mixtures differ from all other stops in that in the manuals they break back; that is, they repeat at intervals of usually either an octave or a fifth. This is since they always sound at a high pitch, and a point is soon reached at which further increases in pitch would be impractical (or the result would be inaudible).

All mixtures are principal-scale. They are often called simply Mixture; other names include Plein jeu, Scharf, Cymbal, and Fourniture (the word for "mixture" in French). For a full list, see Mixture (Encyclopedia of Organ Stops).

The number of ranks is often shown by a Roman numeral; for example, Mixture IV denotes a 4-rank mixture. It may be variable, increasing from the lowest to the highest notes of the keyboard; for example, Plein Jeu III-VII has from 3 to 7 ranks.

If a pitch is specified (for example, Mixture IV 2'), it refers to the pitch of the lowest-sounding rank when used with the low C of the keyboard. Sometimes the mixture label indicates the intervals above the fundamental that will sound; for example, Mixture 15.19.22.26 sounds intervals of a 15th, a 19th, a 22nd, and a 26th (or 2' + 1 1/3' + 1' + 2/3').

4.3.3 Cornet and Sesquialtera

Both these are multi-rank stops; however, unlike mixtures, they are flute-scale and do not break back. They may be present only in the upper part of the manual.

Cornet (pronounced "cornay", has nothing to do with the modern brass instrument) comprises five ranks giving the first five pitches in the harmonic series: 8', 4', 2 2/3', 2', 1 3/5'. It may thus be used alone, and with a soft but prominent sound, may dominate the registration.

If the stop contains fewer than five ranks, one or more other similar stops must be drawn with it to complete the series; hence Cornet IV requires an 8' flute to be drawn, and Cornet III requires both 8' and 4' flutes.

Sesquialtera usually comprises two ranks at 2 2/3' and 1 3/5' (in other words, the non-octave pitches of the Cornet). It may sometimes be of principal scale.

4.3.4 Undulating

Comprises two similarly-voiced ranks, one tuned slightly sharp or flat of the other. When played together, a beat frequency effect is produced. The stop may be either two-rank containing both detuned and normal ranks, or else contain only the detuned rank and require another similar stop to be drawn with it.

Unda Maris is an undulating flute stop. Aeoline and Voix céleste are undulating string stops.

4.3.5 Resultant

Provides a stop of lower pitch than the ranks involved, using difference tones. For example, a Gravissima 64' stop can be produced from 32' and 21 2/3' ranks (only two organs in the world have a true complete 64' rank).

4.3.6 Toy

Unlike all others in this section, these are accessory stops, not speaking stops, as they do not involve pipes. However, unlike other accessory stops (couplers and tremulants), they produce sounds, which include percussions (melodic such as Chimes, and non-melodic), Bird Whistle, and others. They are especially abundant in Theatre Organs (from which the term "toy" originates), but even baroque organs may feature this type of stop.

4.4 Temperament

Temperament refers to adjustments to deal with discrepancies that occur between:

the natural (pure) intervals given by the harmonic series, and
the octave divided into 12 fixed semitones, as on a keyboard

It is a compromise that manipulates pure intervals to make them fit the 12-note octave; if inappropriate, it can lead to obvious sour "wolf" notes. It applies to keyboard and fretted instruments, in which Tuning gives a fixed set of 12 semitones. It is not relevant to singers and players of most other instruments, who can make pitch adjustments on the fly (this is referred to as intonation).

The octave is always untempered (pure), with a constant frequency ratio of 2. However, other intervals depend on the temperament used. The two basic intervals considered (here, and historically) are:

Perfect Fifths: with a pure frequency ratio of 3/2 (3rd harmonic over 2nd)
Major Thirds: with a pure frequency ratio of 5/4 (5th harmonic over 4th)

Temperaments involving these intervals also affect the perfect fourth (4/3, the remainder of the octave after taking out the perfect fifth), and the minor sixth (an octave minus a major third). This extends to other intervals (for example, a tone is half a major third).

Mean-tone temperaments are aimed at preserving the major thirds (and thus also tones). Pythagorean temperaments are based on perfect fifths (and thus perfect fourths). Except for equal temperament, the intervals differ with the key; thus "wolf" intervals may occur with some keys, while other keys give more pleasing results.

To understand why temperament is necessary, we compare the frequency ratios from pure intervals with those that arise from the 12-note octave.

4.4.1 Perfect Fifths

The circle of fifths is generated by starting with a note (here C), and successively adding perfect fifths. This gives the following sequence:

C, G, D, A, E, B, F♯, C♯, A♭, E♭, B♭, F, C

The cycle is thus complete after stepping through all 12 notes, when the sequence returns to C, but 7 octaves higher.

The frequency ratio of this 7-octave interval is 2⁷, or 128. However, this interval is also 12 perfect fifths. If these are pure, the ratio is (3/2)¹², or about 129.75.

As pure perfect fifths are too large to be congruent with the octave, they must be tempered downwards.

4.4.2 Major Thirds

Successively adding major thirds to the note C gives:

C, E, A♭, C

The cycle is thus complete after stepping through 3 thirds, when the sequence returns to C, one octave higher.

The frequency ratio of this octave interval is 2. However, this interval is also 3 major thirds. If these are pure, the ratio is (5/4)³, or about 1.9531.

As pure major thirds are too small to be congruent with the octave, they must be tempered upwards.

4.4.3 Equal Temperament

This is the standard used almost universally today, except in performance of early music. It is unique in providing the same intervals in all keys. This is achieved by dividing the 12-note octave into exactly equal intervals (frequency ratios). Thus the ratio of frequencies in successive semitones is 2^1/12, or about 1.05946.

So in equal temperament, the ratio of a perfect fifth (7 semitones) is 2^7/12, or about 1.4983. This is less than the pure ratio of 1.5, but is actually quite close. However, the major third, being 2^4/12 or 2^1/3 (about 1.2599) deviates much more from the pure ratio of 1.25. Equal temperament thus provides quite pure perfect fifths and fourths, at the expense of major thirds and other intervals.

Although many historical organs and some modern ones use unequal temperaments, this does not usually result in conflicts, as they are rarely played with instruments tied to equal temperament (such as the piano and fretted string instruments).

4.5 Tuning

The pitch and temperament of the organ are determined during its construction; it would not normally be practical to change these later. So tuning is confined to correcting deviations from the original pitch and temperament. Nowadays, equal temperament with a pitch of A=440 Hz is the norm, although even some modern instruments use unequal temperament.

Tuning an organ is a critical and involved process. All the perhaps thousands of pipes in the organ must be in tune with each other; those out of tune produce a noticeable beating effect. Each pipe is adjusted to achieve zero beating with the pipe to which it is tuned. A fixed tuning keyboard is often provided to sound the required pipes, and an electronic tuning device is used to generate the required frequencies. Other equipment required is ear protection, as some pipes may be very loud.

The tuning process begins with a tuning stop, to which most of the other stops will be tuned in turn. This is usually a principal stop at 4' pitch. The middle octave is tuned first. Then the rest of the stop is tuned to itself in octaves, working outwards to the ends of the keyboard. Once the tuning stop is in tune with itself, it is used as a basis for tuning most of the remaining stops.

Adjustments are made by tapping the tuning mechanism of the pipe with a tuning knife, to avoid touching the pipe with the hands. There are several means of adjustment, depending on the construction of the pipe. For example, metal open flue pipes usually have a sliding collar, and for stopped flues the stopper is moved up or down. Different methods apply to reeds, all of which may also affect the tonal properties; thus tuning reeds is more problematic than tuning flues.

Organ pipes are very sensitive to changes of temperature; so much so that the body heat of the organ tuner can affect the tuning. This is since the velocity of sound increases with temperature, and with it the frequency at which the air vibrates. Flue pipes are affected more than reeds, which results in flues and reeds being out of tune with each other (for which reeds usually get the blame, as they are generally more troublesome). In addition, wooden pipes are affected by humidity. It is therefore desirable to maintain even temperature and humidity to reduce pitch fluctuations, and to minimize the frequency with which tuning is necessary.

5 Actions and Wind

Wind refers to the pressurized air fed to the pipes, which stand on one or more wind chests. Actions are concerned with directing the wind to the required pipes by opening and closing valves. The organ contains two separate actions that allow selection of both notes to be played, and the stops to be engaged for each note. Either action may be mechanical, electric, or pneumatic.

5.1 Stop and Key Actions

The two mechanisms that control switching of the wind supply to the pipes are:

stop: opens the airway to one or more complete ranks (operated by stop controls).
key: selects pipes within each open rank (operated by the manuals and pedalboard).