Pipe organ stops
Periods and aesthetics  

Alsacian organ builders

Organs in Alsace


French version Version franšaise
 How does a slider and pallet chest work?

Pipe organs architecture

Section of a classical structure organ, 
with mechanical action.
Great, choir, integrated keydesk, wind-chest, suspended action,
stickers, squares

The following figure shows a section of a classical structure organ with 2 manual keyboards and independent pedal, constituting 3 divisions, altogether form an esthetically classical organ.

The great, or great-organ is played on the upper keyboard, and holds the main stops (for example, the open 8' flue stop). The lower keyboard controls a smaller, independent organ, located at the edge of the organ gallery, behind the organist, and called choir organ (or chair organ) or positif, in this case "positif de dos" ("RŘckpositiv" in German).

The equipment containing the pallets, located under the pipes, decide if the wind does or does not enter the foot of each pipes. They are called wind-chests.

Here, the keys of the great division have their pivot point at the farther end, and directly pull a pallet, enabling the wind to enter in a space located just below the pipes, and called key channel. There is, in each wind-chest, one key channel per key, common to all the pipes corresponding to this note, for each stop. From the key channel, the wind goes in the feet of each pipes whose slider is drawn. If the stop is not draw, the slider holes do not correspond to holes drilled in the upper part of the wind-chest, and the wind cannot blow from the key channel to the pipe.
The key is "suspended" to the pallet, and the pallet spring holds the key up when not played. It is the best notes action, the most accurate, which enables the organist to perform the phrasing.
This is called mechanical suspended action.

With a choir organ, placed as on the figure, it is not possible to have a suspended action the same way as for the great division. There is a sort of seesaw, and the the pallet is pushed by a stickers.

Then, in the figure, the pedal division wind-chests pallets are pulled via a couple of squares. Such an action is generally "heavier" (and less reliable) than a suspended action.

The example of the organ in RibeauvillÚ (the background images) :


 The various divisions
the various sound planes

To each keyboard (and to the pedal) of an organs, corresponds a "sound plane", i.e. a given position, resulting in a characterized acoustic effect.
The Classical aesthetic favors vertically disposed wind-chests, spread from the bottom to the top of the organ case(s).
The Romantic aethetic prefers a back / front disposition.

  • The great division corresponds to the main keyboard. Its wind-chest(s) is (are) placed just behind the front pipes : being given a strong sound and a "proud" position, this division is the main sound plane for the music being played on the instrument.
    The disposition includes the principals (16' if available, 8', 4', 2'), the stopped flues and the flutes (16', 8' and often 4'), the main mixtures, the main trumpet and possibly a clarion. A great, 5 ranks cornet, and maybe a soloist reed (like a Vox humana) usually completes a classical great ("grand orgue") disposition.
  • The choir is sometimes called positif ("Positiv") for it got this name from the small organs with one division. To this keyboard corresponds, in the classical tonal design, a complement to the great organ. Smaller, located in a separate little chest, just behind the organist, it is called positif de dos.
    The pipes - and thus the sound - are nearer to the audience.
    The disposition generally includes a cornet cornet, split in its five ranks, a sharp mixture and normally a soloist reed stop, like the cromorne.
  • The echo division, is located higher in the chest, or simply in its base. The purpose of this sound plane is explicitly said by its name.
    To this keyboard corresponds, in the classical tonal design, a partner to the great organ.
    The disposition generally includes a cornet cornet (sometimes with the three 2'2/3, 2' and 1'3/5 ranks, the bourdon 8' and flute 4' being independent). An "echo" trumpet normally completes the echo's stops disposition.
  • The swell ("rÚcit") is often expressive, and took over from the choir organ in the Romantic tonal design. The swell is located (deep) into or behind the main chest.
    The disposition includes soloist stops like the oboe, and is often grounded on a narrow stopped flue called quintaton.
  • The choir organ, when located just above the keydesk, but in the main chest, is called Brustwerk. It characterizes a Nordic tonal design.
  • A positif, located at the top of the chest (thus in a very special, distant, sound plane) is called "Kronwerk" or crown positive ("Positif de couronne").
  • A special division, specially designed to include the bombarde (but also the 8' and 4' trumpets, plus the accompanying stops) is called "Clavier de Bombarde".
  • Another special division consists in pipes located very far of the main chest, maybe at the other end of the nave. This division is called antiphonal ("Fernwerk").
  • Last, but not least, the pedal organ disposition is often characteristic of the choices in the tonal design.
    In the French Classical organ, the pedal has a limited compass and small keys which do not enable virtuosity. Moreover, the pedal did not contain the deepest stops in the organ (the 32' was often devoted to the great, the pedal being often grounded on a 8').
    The German type pedal has a larger compass (2 1/2 octaves) and an even larger tonal range (from 32' to 2', sometimes with a mixture). Its main purpose is to play the bass, and thus the pedal contains the lowest stops in the organ (see the classical disposition of the organ)..
Divisions disposition
The classical French organ has a "front to back" disposition for the divisions :
  • a positif de dos is very near to the audience
  • the great is just behind : higher, and thus, seen from the nave, exactly behind the positif. The largest front pipes belong to the great.
  • the pedal is behind the great, or even in a separate enclosure (this is sometimes called (pedal "Ó la Silbermann")
The Nordic organ rises its divisions vertically :
  • a "Brustwerk" or chest division is located just above the keydesk
  • an "Oberwerk" or main division comes just above
  • a "Kronwerk" or crown division comes, as said by its name, crowns the whole work.
  • the pedal is placed in two towers, left and right of the main chest, and usually separate. This organ "shows" its pedal pipes.
From ppp to fff...

A lot of devices have been designed to enable the organ stops to play more or less loud. Some of these used the variation of the wind pressure, but they never gave complete satisfaction, because a pipe is tuned and voiced for a given pressure, and thus, changing the pressure gives poor sound effects.

The register crescendo pedal, or simply "crescendo pedal" draws or pushes back the stops one by one. Their only real interest is that they enable to study the way in which the stops are drawn, and thus to know about registration at the time the crescendo pedal was built.

The most widespread system consists in placing a whole division in a box, which can be (smoothly) opened and closed by shades which are controlled by a pedal on the console.
This is called an expression box, and was very much used in the 19th century organs. Generally, the swell of these instruments is expressive, and sometimes the choir organ too.

The wind supply
wedge-shaped bellows, parallel reservoir, wind conduit, postage

The wind supply has to produce the wind, to regulate the pressure and to stock the wind when not used. Thick pipes, called wind conduit bring the wind to the wind-chests, from where it is then dispatched to the pipes.

The difficulty is to keep a constant pressure, although the instrumental part uses more or less wind according to the number of notes and stops which are currently played.

Wind was formerly produced by wedge-shaped bellows, which were in fact (very) slightly modified fireplace bellows.
  • An inlet valve (blue) opens when the inner pressure is lower than the atmospheric pressure (because the bellows are pumped up). The valves lets the air come in.
  • An exhaust valve (green) has the opposite purpose : it is closed when the inner pressure is lower than the atmospheric pressure. (Without it, the wind in organ would fill the bellows).
  • When the bellow is pushed down, the inlet valve closes, preventing the air from escaping to the outside, and
  • the exhaust valve opens, because the pressure in the bellow is higher than the pressure in the organ.

Obviously, as the bellow is either pumped up or pushed down, at least two bellows were needed in order to have a steady pressure.

Until the 15th century, there was no reservoir : the bellows fed directly the wind-chests. Obviously, the wind was not so steady.

This had two major drawbacks :

  • the pressure in the wind-chests depended directly from the the force exerted on the bellows (the operators blowing the bellows had to be very steady in their effort)
  • the lack of reservoir involved having a lot of bellows

After the 15th century, some rigidity has been given to the bellows, and, by putting a weight on them, it was possible to fix the inner pressure, as this weight would substitute for the operator during "emptying" phase. However, the operator had to give a great effort in order to pull the bellows up, as the weigh was added to the effort necessary to let the air in.
Then, the reservoir was introduced. It was first an additional wedge-shaped bellow (yellow), weighed by a ballast or a spring, inflated by the other, regular bellows. The whole was usually stacked vertically. The variation of pressure were corrected by the reservoir-bellow, as it was always weighted and approximately inflated the same manner.

In year 1677, Christian FOERNER invented a very useful accessory : the wind meter, which enables to measure very accurately the pressure. Actually, the pressure has not only to be very even, but has to keep a given value in order to keep a good voicing.

But the former approximately, about the pressure being even in the wedge-shaped reservoir, is important, because in a wedge-shaped reservoir, the pressure does depend on the way it is open.
The parallel reservoir was invented in the 19 th century. Even with a little amount of air inside, the pressure is the same as when it is full inflated. This is because the pressure directly linked to the weigh of the ballast (bricks, railroad rails...) on the reservoir. (The inner pressure is equal to the atmospheric pressure, plus the weight divided by the upper surface). The folds are kept parallel by a system of articulated bars (pink), that have a "Z" shape, and which two extremity, as well as the center of the middle bar are attached to the folds (red pivot points).

However, parallel reservoirs did not immediately give complete satisfaction, because there were too much folds directed to the inside, and they would produce an over pressure when the reservoir was nearly empty. The English clock-maker CUMMINS inventor in 1814 the modern reservoir, when he got the idea to put as many entering as outgoing folds.

Up to the invention of the electric blower, the reservoir was fed by wedge-shaped bellows (then called pumps), operated by levers (either pulled by hand or by the feet).

Parallel reservoir coupled to an electric blower

Nowadays, an electric blower (green) is obviously user to feed the reservoirs. In the 19 th century, when the organ had to be representative of the industrial age, blowers were powered by steam machines.
The blower is a fan revolving in a circular box. The air is sucked in the center, accelerated by the fan, and goes out at the periphery. Its speed gives sufficient pressure to enter in the reservoir.

With the modern electric blowers, it would be possible to blast a reservoir if there would not be a regulator device. It takes the form of a regulator box (blue). It is a frame of shades (red) : their position is controlled by a string pulled by the upper part of the reservoir (through two pulleys). When the reservoir is full, the shades are closed. The blower runs "free", as it cannot expel the air. When the organ plays, the top of the reservoir lowers slightly (but the pressure stays even). The string opens the shades, enabling the blower to blow into the reservoir.

Other small reservoirs, called concussion bellows are sometimes present in the organ itself (near the wind-chests), in order to prevent the wind from "shaking".

The pressure and amount of wind provided by the wind supply is very important. Wind has to be steady, because all the pipes are voiced for a given pressure. When the pressure is artificially increased, the organ sound louder, but, as it was not designed for this pressure, it looses its tuning, and some pipes began to "octaviate", i.e. play an octave too high, as a result of the modification of the vibration mode in the pipe.

Sometimes, it is necessary to bring the wind to pipes which are not directly put on the wind-chest. Some additional pipes are needed.

This is the case with front pipes ("montres"), often with great cornets (to give them a proud position in the chest) and sometimes with large pipes that cannot be held on the wind-chest.


  There is a Zimbelstern in Marienthal

Besides the "real" stops, there are some "non-speaking" stops on the organ consoles :

The couplers enable to play a division from another keyboard than the usual one. It enables two or more divisions to be played simultaneouly from the same keyboard. If the "choir to great" coupler is enabled, each note played on the great will also be played by the choir division. (On the other hand, if the choir keyboard is played, the great does not).

In the dispositions descriptions, if the second keyboard can play the first division, the following code appears : I/II (II plays both, I plays alone).

Keyboards are numbered from the lower to the upper.

The oldest coupler device is the sliding keyboard. It is still widespread among mechanical action, "classical" organs with a choir organ : the corresponding keyboard is the lower one (it is called "the first", and numbered "I"), and the great organ keyboard is just above (and numbered "II").
To enable the coupler, the great keyboard can be slid forward (right drawing). The keys of the lower keyboard (choir) have a bulge on the top, so that in this position, each key of the great keyboard rests on its corresponding key of the choir keyboard. When the great keyboard is played, the choirs' keys are driven simultaneously.

I/II (Sliding keyboard)

A manual / pedalboard coupler has often special names ("Tirasse" in French).
The abbreviation for this is for example I/P (pedal playing the division corresponding to the first manual) (resp. II/P, etc...).

Some small organs do not have any stop specific to the pedalboard (they have no pedal division). The pedalboard always plays the notes on the great (or the single) division. This is called permanent pedal coupler".

As soon as non-mechanical actions have been invented for stops (pneumatic, electric...) it became possible to call stops combinations corresponding to given volumes (pp, p, mf, f, ff, tutti). This is called a preset. The study of presets is often interesting, because they teach a lots of things about the way of registering at the time of their design.

A (free) combination is a way to select a set of stops, and to call them later. This is done with a little lever near the stop tilting button, or by turning the register, or with completely electronic devices.
A piston or toe stud is then used to call or cancel the combination.

The most common preset is the tutti, calling all the stops (or almost).

There are also mechanical devices enabling combinations, but these devices are often limited to small organs. In order to make the registration easier on large instruments with mechanical stops action, but also in order to enable different wind pressures, the wind-chests are sometimes split in two or three parts :

Thus, it is possible to cut the wind supply of the mixtures or the reeds. This is called "reeds canceling" or "mixtures canceling", or reciprocally "reeds call" or "mixtures call".

If the wind pressure stability is essential to get a sustained, regular sound, it is also possible to vary it on purpose, to get a "vibrato" effect. This is done by the tremulant (or tremolo). It is designed to be used with soloist stops (a vox humana for example, shall always sound properly with the tremulant).
  • The strong tremulant (or "lost wind tremulant") is a pallet that produces a periodical leakage of wind on the wind pipe to the wind-chest. The strong tremulant is used in the classical organ, especially to give additional expression to the reeds, in the grand jeu.
  • The soft tremulant (or "in the wind") is simply a device which revolves in the wind pipe to the wind-chest.

Some tremulants concern the whole instrument, others have an action limited to a division. The tremulant is a rather ancient devices (it appeared in the 16th century).

A nightingaleis a set of (generally 3) pipes, which are partly immersed in a water box. The device produced a sound which is very like the bird's song. Very common in the old times (it appeared in the 16th century), it is now very seldom. This "musical toy" was adapted to organs not designed for virtuoso performance, but to give to the audience very differentiated sounds and effects.

A Zimbelstern is a set of bells, usually taking the form of a crown of a star. These devices were extremely popular in the Middle-Ages and the Renaissance. They were usually completed by automatons and animated puppets.

Until the 17 th century, the organs were often built with percussion stops : chimes, spherical bells, drum. Today, they have also nearly completely disappeared.

The organ in the cathedral of Strasbourg has two animated puppets, one each side of the pendentive (one plays the trumpet, the other has a drum). They can be moved from the keydesk.

The short octave is not a device, but a key disposition enabling to spare 4 large (expensive) pipes from the first octave. This disposition has been used until the 17 th century (and even later, almost in Italy). The lowest octave lacks the C#, D#, F# and G# :
Octave courte

For a very long time, even during the 18 th century, the manual keyboards compass was 4 octaves (49 notes). A compass of 44 or 45 notes is characteristic of a short octave keyboard.
Many organs have been built with 48 notes keyboards (lacking the lowest C#, and its key)