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; each can be "on" (admitting the passage of air to certain pipes), or "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. On electric or electronic organs that imitate a pipe organ, the same terms are often used, with the exception of the Hammond organ and clonewheel organs, which use the term "drawbar".
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. The phrase "pull out all the stops" has entered general usage, for deploying all available means to pursue a goal.
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. A stop may be linked to a single rank, or to multiple ranks. While nowadays one speaks of "drawing" a stop to select a particular rank or set of ranks, the earliest organs were constructed with all ranks "on" by default.
The mechanism for operating 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, they operate 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, Spain or Portugal, 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) generally 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.
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. Many organs originally built with mechanical actuators have been retrofitted with electric actuators.
The term unification refers to the practice of expanding the tonal resources of an organ without adding more pipes by making notes available to different stops 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 by higher and/or lower stops. Unification and borrowing (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.
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.
While unification, borrowing (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. 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. Nonetheless, heavy unification can create issues for visiting artists with limited practice times, or those improvising compositions. Also, in a "straight" organ, voices are scaled for their intended job. As an example, the octave (4') diapason is generally of a smaller scale and softer than the 8' diapason rank it goes with, whereas in unification they would be the same strength from the same set of pipes. Straight reed choruses (16', 8' and 4') have the luxury of different sounding component ranks, whereas a unified reed chorus has voices that are identical, since they come from the same set of pipes. Further, there would be missing pipes on unified choruses in chords where an octave is sounded.
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 are 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 large 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. Borrowing 16' manual ranks for the pedal division is more widely employed because of the expense and space requirements of 16' stops and the versatility this allows.
The pitch produced by an organ pipe is a function of its length. All else equal, 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 ("half 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 ?.
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?, ? (labeled 3? on some German and Swedish organs), 2?, and ? (or ? on some German organs).
The sounding length of a mutation stop gives the answer as to what pitch the rank sounds. For example, a stop labeled ? (or one-third of 8?) sounds at three times the frequency, that is, the interval of a twelfth above unison pitch. This third harmonic (twelfth, quint, qvinta, rorkvint, or nazard [nasard]) is the most-common pitch, followed by the fifth harmonic (tierce [terz or ters on some organs]) (?) and sixth (larigot, nasat) (?) and there are rarer examples from higher in the series, such as the "septième" or "septima" (?) 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., quint-bass ?. 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 (e.g., Bourdon 32?), saving the space and money otherwise needed for larger bass pipes; such an effect is termed a resultant.
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:
This is an example of a pipe organ stoplist, showing both common stop names and conventional formatting. (Within each division, flues are listed before reeds, then low to high pitch, then louder to softer stops within a pitch level. Separate celeste stops are next to their corresponding normally-tuned stops. Reed stops are often labeled in red on stop knobs or tabs.)
8? Prestant I-II
8? Chimney Flute
4? Harmonic Flute
2? Super Octave
2? Night Horn
8? Trompette en Chamade
Swell to Great
8? Open Diapason
8? Stopped Diapason
8? Vox Céleste
4? Röhr Flute
2? Block Flute
16? Contra Fagotto
8? Vox Humana
32? Contra Violone
16? Open Diapason
16? Sub Bass
16? Lieblich Gedeckt
4? Choral Bass
II Rausch Quinte
32? Contre Bombarde
Great to Pedal
Swell to Pedal
Organ pipes fall into five broad categories:
Percussion stops (often referred to as "toy counters" or "toy stops"), unlike other organ stops, are not aerophones, but actual embedded percussion instruments (although they may still be actuated by the wind supplies of the organ). Both tuned and untuned percussion stops exist (for instance, marimba and snare drum, respectively). They are commonly designed to imitate orchestral or band instruments, or to imitate non-musical sounds (for instance, thunder), or to produce unique sounds (for instance, zimbelstern). Percussion stops are particularly common in theatre organs, which were generally made to accompany silent films.