An organ stop (or just stop) is a component of a pipe organ that admits pressurized air (known as wind) to a set of organ pipes. Its name comes from the fact that stops can be used selectively by the organist; some can be "on" (admitting the passage of air to certain pipes), while others can be "off" (stopping the passage of air to certain pipes).
The term can also refer to the control that operates this mechanism, commonly called a stop tab, stop knob, or drawknob.
The term is also sometimes used as a synonym for register, referring to rank(s) of pipes controlled by a single stop. Registration is the art of combining stops.
Organ pipes are physically organized within the organ according to note and timbre, into sets. A set of pipes producing the same timbre for each note is called a rank, while each key on a pipe organ controls a note which may be sounded by different ranks of pipes, alone or in combination. The use of stops enables the organist to selectively turn off ("stop") certain ranks in order to produce different combinations of sounds, as opposed to hearing all sounds simultaneously. While nowadays one speaks of "drawing" a stop to select a particular rank, the earliest organs were constructed as blockwerk with all ranks 'on' by default.
The mechanism used to operate the stops varies widely, but the principle is the same: the stop control at the console allows the organist to select which ranks of pipes will sound when a key is pressed. When the organist desires a rank to sound, he or she operates the corresponding control at the console, allowing wind to flow to the pipes. Likewise, the organist can deny wind to the pipes by operating the same control in the opposite direction. Common stop controls include stop knobs, which move in and out of the console, and stop tabs, which toggle back and forth in position.
Some organs, particularly smaller historical organs from England or Spain, feature divided registers, in which there are two stop knobs for certain ranks. One stop knob will control the upper portion of the keyboard, and the other will control the lower portion of the keyboard. This arrangement allows the upper portion of the keyboard to sound a different registration than the lower portion, which lends a greater versatility to smaller organs, especially those with only one manual.
Ranks which are neither divided nor extended (see below #Unification and extension) contain as many pipes as there are keys on the keyboard to which they are assigned: in most cases 61 pipes for a rank assigned to a manual and 32 pipes for a rank assigned to the pedal.
Methods of actuation
Over the course of the history of the pipe organ, there have been several different designs by which stops are actuated. In the longest-standing design, known as the slider chest, there is a strip of material (typically wood) called a slider which fits underneath a given rank of pipes. The slider has small holes drilled in it, one for each pipe in the rank. When the stop is set such that pipes are inactive, the holes are misaligned with the pipes, preventing the air from flowing up into the pipes above. When the stop is set such that the pipes are active, the slider moves over, aligning the holes with the pipes, allowing air to reach them. Because the slider chest was developed before the advent of electricity, it is inherently mechanical in nature. However, it has been adapted to operate with electricity as an actuating component.
Unification, Duplexing, and extension
The term unification refers to the practice of expanding the tonal resources of an organ without adding extra pipes by deriving multiple pitch levels from the same rank of pipes. For example, an 8' Gedeckt may also be made available as a 4' Gedeckt, either on the same or on a different manual. Borrowing or duplexing refers to one rank being made available from more than one stop knob, often on different manuals or pedal.  Extension refers to the addition of extra pipes to the high and/or low ends of a rank in order to allow that rank to be borrowed at a higher and/or lower pitch. Unification and duplexing is mostly related to pipe organs with physical pipes; however, some (older) electronic organs also used unification and duplexing to expand the tonal resources of a limited number of synthesized virtual ranks.
While unification, duplexing, and extension increase the tonal resources and flexibility of the organ, the downside is that greater care needs to be taken by the organist in registering the organ, particularly when the composition requires many notes to sound at the same time. Because stops at multiple pitch levels or from multiple coupled divisions may be engaged at the same time for the same note, it is possible for these stops to interact in unexpected ways when more than one note sounds at the same time (as in a chord). For example, playing middle C (C4) together with a C two octaves higher (C6) on an organ registered with an 8' Gedeckt and a unified 4' Gedeckt will result in the intervening C (C5) not sounding when played with the other two notes. In this case, only four total pipes will sound instead of the six pipes that would sound on a non-unified organ. Playing with full registration (all the stops out) on an organ that is heavily unified and duplexed may result in an occasional chord that sounds thinner or that emphasizes higher harmonics more on some notes than on others. Part of an organist's training is to detect unification and duplexing and to create registrations that take unification and duplexing into account. 
Borrowing between manuals occurs in English organs from about 1700, but extension of pipe ranks for the purpose of borrowing at different pitches is a relatively recent development. Extension and unification were heavily used in theatre organs to produce the maximum number of voices from a minimal number of pipes. It is still typical to see a significant amount of unification and duplexing in practice organs and small church organs. Traditionally, less use has been made of extension in church organs and those designed for classical music, with authorities tending to regard borrowing in general and extension in particular as things to be avoided if possible, except in a few cases where space for pipes is limited, making extension and/or unification necessary.
If the concepts of unification, borrowing and extension as explained in this section are difficult to understand here is a clearer explanation taken from here:
Sometimes, a single rank of pipes may be able to be controlled by several stops, allowing the rank to be played at multiple pitches or on multiple manuals. Such a rank is said to be unified or borrowed. For example, an 8′ Diapason rank may also be made available as a 4′ Octave. When both of these stops are selected and a key (for example, c′) is pressed, two pipes of the same rank will sound: the pipe normally corresponding to the key played (c′), and the pipe one octave above that (c′′). Because the 8′ rank does not have enough pipes to sound the top octave of the keyboard at 4′ pitch, it is common for an extra octave of pipes used only for the borrowed 4′ stop to be added. Such a rank is said to be extended. In this case, the full rank of pipes (now an extended rank) is one octave longer than the keyboard.
Pitch and length
The pitch produced by an organ pipe is a function of its length. Longer pipes produce lower pitched notes, while shorter pipes are higher in tone. An organ stop utilizes a set (rank) of pipes of graduated lengths to produce the range of notes needed. Stops having ranks of pipes that are sized and tuned to sound the pitch normally associated with the keys (i.e. the pitch of same keys on a piano) are called "unison stops". Other stops use pipework which is longer or shorter than that of unison ranks to speak at a fixed interval above or below unison pitch. ("octave pitch" or "mutation pitch")
The pitch of a rank of pipes is denoted by a number on the stop knob. A stop which speaks at unison pitch, or "native pitch" is known as an 8' (pronounced "eight foot") stop. This nomenclature refers to the approximate length of the longest pipe in a rank of open pipes. In a rank of stopped pipes, the lowest pipe is about 4 feet long, but because it sounds at unison pitch, it is also known as an 8' stop.
The octave sounded by a given pipe is inversely proportional to its length ("1/2 the length = double the pitch"), meaning that a 4' stop speaks exactly one octave higher than an 8' stop. Likewise, a 2' stop speaks exactly one octave higher than a 4' stop. Conversely, a 16' stop speaks exactly one octave below an 8' stop; and a 32′ stop speaks exactly one octave below a 16' stop. Octave pitch lengths used in actual organs include 64', 32', 16', 8', 4', 2', 1', and 1/2'.
Ranks that do not speak at the unison or some octave of the unison pitch are called mutation stops (or, sometimes "aliquots"). They are rarely used on their own; rather, they are combined with unison stops to create different tone colors. A typical and distinctive sound of the organ is the cornet, composed of a flute and ranks making up its first four overtones, sounding 8', 4', 2⅔', 2', and 1⅗'.
The sounding length of a mutation stop gives the answer as to what pitch the rank sounds. For example, a stop labeled 2⅔' (or one third of 8') sounds at three times the frequency, that is, the interval of a twelfth above unison pitch. The third harmonic (twelfth, quint or nazard) is the most common pitch, followed by the fifth (17th or tierce) and sixth (larigot) but there are much rarer examples from higher in the series, such as the "septième" and "none".
Mutations usually sound at pitches in the harmonic series of the fundamental and, except where they are derived from unit ranks, are always tuned pure. In some organs, lower pitches are used to create difference tones e.g., Quintbass 10⅔'. Such 'helper ranks" that sound at the fifth just above or fourth below the fundamental (e.g., Bourdon 16'), can create the impression of a stop an octave lower than the fundamental (i.e., Bourdon 32'), saving the space and money otherwise needed for larger bass pipes.
Certain stops called mixtures contain multiple ranks of pipes sounding at consecutive octaves and fifths (and in some cases, thirds) above unison pitch. The number of ranks in a mixture is denoted by a Roman numeral on the stop knob; for example, a stop labeled "Mixture V" would contain five pipes for every note. So for every key pressed, five different pipes sound (all controlled by the same stop).
Pipe ranks have particular names, which depend on a number of factors ranging from the physical and tone attributes of the pipes in that rank, to the country and era in which the organ was manufactured, to the pipes' physical location within the organ. Each stop knob is labeled with the name of the rank it controls. In general, that label gives the organist two vital pieces of information about the rank of pipes in question:
- Which octave of pitches the rank is natively tuned to
- Which tone quality the rank possesses (for example trumpet, flute, etc.)
Classifications of stops
All audio examples are provided courtesy of the Encyclopedia of Organ Stops, by Edward Stauff.
Stop names are indicative of the tone made by the rank(s) of pipes they control. Organ pipes fall into five broad categories:
- Principal (or Diapason)
- Listen to example
- Principal stops are non-imitative; that is, their sound does not attempt to imitate that of a particular instrument. The Principal sound is the one sound which is unique to the pipe organ; it is the sound which comes to mind in the context of traditional church music (such as hymns). While spellings and names vary by language and era, here are some common examples:
- Principal (also Diapason, Open Diapason, or Montre)
- Super Octave (also Doublette or Fifteenth)
- Twelfth (sometimes in the Flute category)
- Mixture (also Fourniture, Plein Jeu, Cymbal, or Sharp followed by a Roman Numeral; example: Mixture III, or Fourniture IV-VI)
- Listen to example
- Listen to example
- Flute (or Flöte)
- Traverse Flute (or Transverse Flute)
- Stopped Diapason (or Stopped Flute) — despite its name, the Stopped Diapason is a flute-class stop
- Gedeckt (or Gedackt)
- Rohrflöte (or Chimney Flute)
- Cor de Nuit (or Night Horn / Nachthorn)
- Spill Flote
- Spitz Flote
- Flach Flote
- Twelfth (sometimes in the Principal category)
- Sifflet (or Sifflöte)
- Octavin (or Flûte Octaviante)
- Flûte d'Amour
- Flûte Harmonique (or Harmonic Flute)
- Sub Bass (or Subbass or Sousbasse)
- Major Bass
- Listen to example
- Gamba (or Viola da Gamba)
- Vox Celeste (or Voix Céleste)
- Listen to example
- Trumpet (or Trompete, Trompette, Trompette en chamade or fanfare trumpet)
- Trombone (or Posaune)
- Oboe (or Hautbois)
- Cromorne (or Krummhorn)
- Vox Humana (or Voix Humaine)
- Listen to example of String + Principal
- Hybrid stops contain one rank of pipes which attempts to combine the tones of two other classifications of stops, such as Principal + String, String + Flute, or Principal + Flute. Common examples:
- Geigen (or Geigen Diapason", Geigen Principal, or Violin Diapason)
- combination of String + Principal
- combination of Flute + Principal
- Listen to example of String + Principal
Notable organ stops
- The loudest organ stop in the world is the Grand Ophicleide located in the Right Pedal division of the Boardwalk Hall Auditorium Organ. It stands on 100″ wind pressure. Rumor has it that the former organ curator always warned the stagehands when the Grand Ophicleide was going to be used, because of the sheer volume.
- The mixture stop with the largest numbers of pipes, called Ple, can be found in Santanyí (Majorca), Spain. It has twenty-two ranks in the left hands and twenty-five in the right.
- There are only two true and complete, non digital, acoustic and down to the sub-sub-contra-C, 64' stops in the world. The Contra-Trombone 64′ in the Sydney Town Hall Grand Organ (click here for a sound sample), and the Diaphone-Dulzian 64′ in the Boardwalk Hall Auditorium Organ (click here for a sound sample). Due to the limitations of most loudspeakers, and the limitations of human hearing, the listener will not actually hear the lowest frequencies in the sample, but will be actually be hearing the harmonics above them.
- Many large organs have a 64′ stop in their stoplist, but nearly all of these are either digital, acoustic imitations-(32′ combined with a 21⅓ extension creating a 64′ impression)- or else a sound sample of a higher-pitched stop electronically altered to sound 1+ octaves lower. The Boardwalk Hall Auditorium Organ is capable of creating a resultant 128' stop by combining its 64' and 42⅔' stops.
- These sound samples start with the 16' CCC then goes down 2 octaves to the 64' CCCCC.
- Some other organs also have a part of the lowest octave, usually the top 3 or 4 pipes.
- There are Percussion (tuned and untuned) stops and it is unknown if they are in the Flute, Principal, String, Reed, or Hybrid category.
- Stevens Irwin Dictionary of Pipe Organ Stops
- George Ashdown Audsley Organ Stops and Their Artistic Registration
- James Dalton, "Iberian organ music before 1700," in The Cambridge Companion to the Organ, ed. Nicholas Thistlethwaite and Geoffrey Webber (Cambridge: Cambridge University Press, 1998), 165.
- "Understanding the Pipe Organ" by John R. Shannon, 2009, p.83
- "Understanding the Pipe Organ" by John R. Shannon, 2009, Chapter 6