The diapason is the basic organ stop.
It is always built with open flue pipes. They are often the front (visible) pipes of the organ. The diapason is then often called "montre" in the French denomination.
If the length of a flue pipe is linked by physical laws to the wanted note (pitch), the width (diameter) is more free. This is called "scale". Diapasons are of medium scale, thus in a good average of the width/length ratio. The scale of a diapason is smaller (it is "narrower") than a flute, but broader than a string.

It is often called "principal", and "Geigenprincipal" when a bit narrower.
Note that sometimes diapasons are called "flutes". This may be sometimes very confusing.
Diapasons are encountered in any pitch :

• in 8', obviously, and then often used as front pipes
• in 4', at the upper octave of the previous. It is then often called "Prestant".
• in 2', again one octave higher. It is then at the fifteenth of the 8', thus called "fifteenth" or "doublette"
• in 1', again one octave higher. It is called "Sifflet" in France.
• in 16', one octave lower than the 8'. It is often located in the pedal of the organ. Having a manual (open) 16' diapason is characteristic of great organs, thus called "16 feet organ". In France, it is often called "Contrebasse".
• and even in 32', one octave deeper than the 16'. Often put on the great keyboard on classical French organs, it is nowadays almost only encountered at the pedal of large instruments.

The diapason is normally built in tin or wood (especially for great pipes).

 
Stopped 
flue pipe

Stopped flues are stops with a soft sound (with few overtones). They are found in almost all organs. The flue pipes that are used to build it are stopped, i.e. covered with a cap. When a pipe is stopped, its sounds falls of an octave. A stopped flue playing in the 8' pitch is actually only 4' high. It is thus less bulky than an open flue. This is the reason why stopped flues 8' are found in even very small instrument, where an open 8' flue pipe could not be held.
It is of broad scale, like a Flute, with which it mixes very well. Often, stopped flues are considered as flutes. But normally a flute has to be open, whereas the stopped flute is always stopped.

Stopped flues are encountered up to 4' :

• in 8', it is present in almost every organ, under the name "bourdon 8'".
• in 16', sounding an octave lower than 8'. When in the pedal, it is often called "soubasse", "Subbass".
• in 32', again one octave deeper, in large instruments.
• in 4', the pipes are not completely stopped. They end with a little additional pipe, looking like a chimney. The stop is then called "flûte à cheminée", "Rohrfloete" in German.

Stopped pipes are never built higher than 4', because they constitute a soft stop, whose role is not to produce overtones. Moreover, building of very small stopped pipes would be very difficult, and their tuning even more difficult.
The "cor de nuit", Nachthorn is a very soft stopped flute (bourdon), one of the less loud of the organ.

Stopped flutes are built in wood (for the largest) or in "common metal", which is an alloy of tin and lead.

The flute has a soft but clear sound, achieved by using flue pipes with broad scales and high mouths. Flute pipes are broader than diapasons.

Often, bourdons are considered as "flute stops". But a real flute is open at the top, whereas a bourdon is stopped. Sometimes, the words "flûte creuse", "Hohlfloete" are used to specify that the flute is open.

Some flutes are conical.
Flutes are encountered in all the sizes :

• in 8', it is sometimes called "flûte majeure", "flauto major", as opposed to the "flauto minor", 4' of the choir.
• in 16', sounding an octave lower than 8'. It is more seldom (because stopped flues are softer and are less bulky), except et the pedal, where it produces and excellent "base sound" for the whole work.
• in 4', sounding one octave higher as the 8',
• in 2'. In this case, it is often called "quarte de nasard", because it is one "quarte" higher than the mutation stop called nasard.
  The romantic period calls "flageolet" the flute 2'.
• in 1', again one octave higher, it is often called "piccolo". But for some organ builders, a piccolo is a diapason.
• Most mutation stops are built with flute pipes. The cornet is also built with flute pipes.

The romantic period loves harmonic flutes.

A good flute is build using wood or "common metal", very rich in lead.

To give overtones to the organ sound, the 19th century sought an other way than the "classical period" (overtone synthesis with the help of mutations and mixtures). While getting rid of bright and high-pitched tierces and cymbals, the romantic and symphonic organ prefers giving timbre to flue stops by building them with a smaller diameter. Pipes with such a narrow scale are called "strings", as they are designed to imitate the sound of string instruments, especially during the attack.

The main string stop is called gamba, or "viole de gambe".

When using a scale half way from the diapason and the gamba (thus having a less stressed "sting" character) is called "salicional". This stop has been very popular, even in small instruments.

"Narrow" stopped flues are called "quintaton" (because the fifth ("quint") of the fundamental sound is very present).

Strings are also built en 4'. A 4' salicional is often called salicet
Strings are often built in 16'. Often called cello ("Violoncelle") at the pedal, it is used to add some "string orchestra hit" attack.

By slightly detuning a string pipe, and playing it together with a normal one, a vibrato effect, i.e. a slow undulation is produced. These stops are called "voix céleste", "Schwebung", and sometimes unda maris, especially when the undulation is very slow. Sometimes, these stops (either really constituted by two string pipes per note, or giving a single, detuned pipe to be played with the salicional or a gamba) is called "aéoline".

The salicional is called "jeu céleste" by some organ builders (Callinet for example), but it does not undulate, and is not to be confused with the "voix céleste".

The "gemshorn" is conical, narrower at the upper end.
Discrete, sometimes very soft, it is often considered as part of the flute stops.

The "dulciana", a very narrow pipe, is the softer string. For the same reason, it is sometimes called "flauto dolce", or "dolce".

The fugara is very narrow, but is cylindrical. Sometimes, it is only the name for the salicional.

These narrow ("small scale") pipes are often built with tin. They are difficult to build, because the very large mouth (compared to the diameter of the pipe), if it produces nice overtones, also has a fuzzy attack. Strings are normally not soloist stops, and they sound better in the bass. To give a better attack, a small round wooden or metallic roll is often placed before the mouth. This is called a pipe beard, or harmonic bridge.

The trumpet ("trompette", "Trompete") is a reed stop whose resonator pipes are conical, the upper part being broader. Their length is the same as the flue pipes producing the same note : they are said to be of "normal" length. There are often more than one trumpet in an organ (of different pitches or put at different keyboards). All the trumpets are called in French the reeds "batterie".

Trumpets are built :

• in 8',
• in 16', playing one octave lower. The stop is then often called "bombarde". It is one of the louder stops of the organ. It is sometimes called "ophycleide". "Posaune" is the German word for the 16' trumpet.
• in 4', playing this time one octave higher than the usual trumpet, it is called clarion.
• very seldom in 2' (because a high-pitch reed pipe is very difficult to build, and does not keep its tuning). For this reason, the smallest pipes of the trumpet (and of course clarion) stops are often replaced by flue pipes.

Sometimes, the pipes of the reed battery is put horizontally, outside the chest. The sound is more direct, and the tuning is better kept. Those stops (and pipes) are said in "chamade".

In romantic and symphonic organs, reeds have been built with double-length (sometime triple-lenght) resonators. Trumpets or clarions having such double-lenght resonators are called harmonic.

A good trumpet is built out of wood or tin.

This stop, often encountered in symphonic organs, is a soloist reed stop, built with resonator pipes which have the same length as trumpets, but narrow at the base, and with a wider cone at the top.

The Oboe stop is often split in two parts "bass" and "treble" ("discant"). There are two stops at the console, to call the two parts independently. The bass is then called "bassoon", or "fagott" and voiced slightly differently. The bassoon has normally a broader resonator than the oboe.

In a reed pipe, the body is only used as a resonator, and does not fix the frequency of the sound. It is thus possible to build reed pipe playing at the normal pitch (8'), but with resonators being given only the half of the normal length, i.e. the half of the flue pipe giving the same note.

By doing that, and giving to the resonator a cylindrical shape, can be built the "cromorne", "Krummhorn", which is a classical soloist stop, with a warm, rich sound. It is often found at the "positif" (choir) keyboard.

When voiced according to the German organ building, the cromorne is called "Schalmei".

The symphonic period (second part of the 19th century) has modified the cromorne (especially its reed), to imitate the clarinet, and thus giving this name to the new stop.

By going farther in the resonator shortening than for the cromorne, very built very distinctive stops, because letting out lots or overtones, and used as soloists.

The "voix humaine" (human voice, or "vox humana") has a very short cylindrical resonator, partially stopped. It is often played with the tremblant.

The "musette" has a resonator built with two opposite cones. Often, the musette has "free" reeds, i.e. the tongue does not hit the shallot while vibrating. The euphone has the same kind of reed, but a longer resonator.

Finally, the "régale" is a reed stop which has a really small resonators, simply consisting in small boxes, with some holes to let the wind get out. Some small organs have been built only with these kind of stops. These instruments were called "regal-organ".

Apart from reed stops described above, which are said to be "", because the tongue hits the shallot while vibrating, some are rather built like an harmonica : "euphon", "english horn". The tongue moves freely in a slot.

To create new sound colors, it is necessary to produce overtones. Any musical sound is a mixture of many notes : the fundamental (for example C), which is also the first overtone, but also its octave (second overtone), the fifth above this octave (third), and many others, always higher.
Some organ stops are used only to produce wanted overtones, and are built with pipes that produce this overtone as their fundamental. So, these pipes give different notes than the key than makes them play
Thus, to give overtones to a C1 (the lowest of the keyboard) played by a 8' pipe, one can want to hear :

• the fundamental or first overtone. The 8' itself gives it.

• The second overtone. It is the Octave, i.e. the C2 in our example. A 4' is played together with the 8'.

• The third overtone. It is the fifth of the latter octave, i.e. G2.
  The stop playing a G2 when the C1 key is stricken is called twelfth, or "quinte". To build the stop, flute pipes are used. It is also possible to use stopped flutes (except for the small pipes, where open, conical flutes are preferred). In this case, the stop is called "nasard" (the word comes from "nose", and it is often written "nazard").

2'2/3

The length of the longest pipe of this stop is thus that of a pipe giving the G2 (which plays here on the C1 key). This length is 2 feet and 2 third :

• The fourth overtone. That is : 2 octaves above the fundamental, C3 in our example. The interval between the overtone and the fundamental is a fifteenth. A 2' stop will produce it. The Principal 2' is thus called "fifteenth" ("doublette"). The Flute 2' sounds one fourth above the nasard. This is the reason why it is called, in French, "Quarte de nasard", or simply quarte.

• The fifth overtone is the third of the previous overtone. An E3 in our example.
  The corresponding stop is called tierce ("Terz"). This stop is very important in the French classical organ. These stops were said to be "chirpy", and were systematically removed from the organs in the late 1800s.
  The tierce is build with flute pipes. The words "jeu de tierce" are other words for cornet. But the cornet normally only begins at F2 or C3, and is called by a single stop. The jeu de tierce plays on the whole keyboard, and is normally split up in four stops : 8' (usually bourdon), 4' (flute), 2'2/3 (nasard), 2' (quarte) and 1'3/5 (tierce).

1'3/5

The length of the longest pipe of this stop is thus that of a pipe giving the E3 (which plays here on the C1 key). This length is 1 feet and 3 fifth :

• The sixth overtone is the fifth of the 2' octave. A G3 in our example.
  The corresponding stop is called larigot.

1'1/3

The length of the longest pipe of this stop is thus that of a pipe giving the G3 (which plays here on the C1 key). This length is 1 feet and 1 third :

• It is possible to go further, but the stops are more seldom. The seventh overtone is the seventh (as a musical interval) of the fourth overtone, i.e. the Bmoll3 in our example. The corresponding stop is called septième (seventh), and its length is 1'1/7 :

1'1/7 Overtone 7 = Septième 1'1/7

All these stops enable to build up various overtone combinations, and this very precisely, as it it possible to choose them one by one.

The mutation stops previously described are "simple", the meaning of the word is that there is only one pipe to play a single note. To hear the fundamental and its 2 first overtones, 3 stops are to be drawn.
But and other way to proceed is to put all the pipes corresponding to chosen overtones together in one stop, so that they play together.

The stop that plays the 5 following overtones on each note is called Cornet V :

• the fundamental (8')
• the octave (4')
• the fifth (nasard) (2'2/3)
• the fifteenth (quarte de nasard) (2')
• the third (tierce) (1'1/3)

This stop is an excellent soloist, who also mixes very well with the "French" reeds of the "batterie", with which it constitutes what is called the "grand jeu".

by extension, the Mutations stops corresponding to the ranks in the cornet are often called "split up cornet" ("cornet décomposé"), as it is possible to build up a cornet by drawing all these stops. The tierce (third) gives its typical sound to the cornet. It is possible to remove the 4' or the 2' in a cornet, but with no tierce 1'3/5, it would not be called a cornet.
This is the reason for which the cornet is often called "Jeu de tierce", which is not to be confused with the tierce stop.

The "cornet III", or 3 ranks cornet lacks the 8' and 4' pipes. These two ranks are in fact put apart, as individual stops (bourdon 8' and flûte 4'). The 8' is to be drawn with the cornet III, and the flute 4' maybe optional.

The "sesquialtera", or 2 ranks cornet lacks the 8', 4' and 2' pipes (i.e. gives only the fifth 2'2/3 and third 1'3/5). It is to be used together with a bourdon 8'. The flutes 4' and 2' are optional. Sometimes, the sesquialtera is not build with flute pipes put with principals.

The cornet does not usually extend to the bass, because it is often used with the reeds (which are louder in the bass), to "re-balance" the organ.
Even played as a soloist, the upper half of the keyboard is the more useful. To play as soloist tenor ("Tierce en taille"), the Jeu de Tierce is to be used, not the 5 ranks Cornet. This is the reason for which the cornet V only begins at C3 or F2. It is thus often called "dessus de cornet".

Limited to the upper half of the keyboard, the dessus de cornet is rather small. It can be put at a special place in the organ chest, where it will be well brought out when used as soloist. For example, it is often just behind the front pipes, a bit higher than the other pipes.

Mixtures are stops for which each note is given by more than one, high pitched pipes (i.e. build with more than one "rank" of pipes). Each rank plays an overtone of the fundamental note, which is not part of the stop.

Mixtures are thus used in addition to fundamental stops, and are their overtones crown. When principals are played together with the mixtures, they constitute the "plein-jeu".

The more common mixture is the "fourniture". It consists in 3 to 7 ranks, giving octaves and fifths to the fundamental.

It is completed with the "cymbal", or "sharp mixture", which has nothing to do with a percussion stop, but is rather a high-pitched fourniture.

There are a lot of decisions to be taken when designing a composed stop : the number of ranks, the pitch of each of them, the progression of the "scale" in each rank (sounding like flutes or more like principals).

Moreover, as the ranks begin at a relatively high pitch (some ranks begin, for example at 1/2', already one octave above the principal 1') and as it is not possible to go higher infinitely (because the pipes would only have a couple of millimeters, would be impossible to tune and would produce a nearly inaudible sound), the mixtures "goes up" as would do a singer who would try to sing always higher notes : it "drops" one octave or fifth at given levels. This is called a "break".

The sesquialtera, especially when built with principal flues, is often said to be a mixture.

The "Rauchsquinte" is the combination of the fifth and its octave (i.e. 2'2/3 and 2').

It is possible to say that the "carillon" is a sort of cornet for which the 2' rank would have replaced by a 1' rank, thus sounding more sharp. It is thus a kind of sharp mixture with a "third" (tierce) rank, and normally without any break.

The cymbal has often its breaks at different level than those of the fourniture that it has to crown. Beginning higher in pitch, the cymbal has normally more breaks than the fourniture. The way to do the mixture breaks is an aesthetic choice and is often typical of a given period or a given organ builder. Ranks, pitches, breaks and their positions are described, for a mixture stop, in a table called "mixture analysis table"

Example of a 5 ranks fourniture :

• Starting with the lowest C of the keyboard : C1
  • A 2' (giving C3, like the fifteenth)
  • A 1'1/3 (giving G3, like the larigot)
  • A 1' (giving C4, like the sifflet)
  • A 2/3' (giving G4)
  • A 1/2' (giving C5)
• This will be true for all the keys between C1 and E2. On the F2, arriving to the "medium" of the keyboard's compass, building the 1/2' rank pipes becomes difficult. Thus, there will be a break here, by dropping each rank by 1 octave (but it could be possible to drop by one fifth) :
  • The 1st rank drops 1 octave, giving now 4'
  • The 2nd also drops 1 octave, giving 2'2/3
  • The 3rd, giving now 2' is the continuation of the first rank, this will "hide the break"
  • The 4th, giving 1'1/3, is the continuation of the second
  • The 5th, giving 1', is the continuation of the third
• Then, at the F3 of the keyboard, a new break can be placed :
  • The 1st rank drops now to 8'
  • The 2nd to 5'1/3
  • The 3rd, dropping to 4' is the continuation of the first rank, this will again "hide the break"
  • The 4th, giving 2'2/3, is the continuation of the second
  • The 5th, giving 2', is the continuation of the third