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.
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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,[1] 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.
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.
Other common designs include the spring chest, the cone valve chest, and the Pitman chest.
The term unification refers to the practice of expanding the tonal resources of an organ without adding extra pipes. Borrowing or duplexing refers to one rank being made available from more than one stop knob. This allows the rank to be played at a different pitch or on a different manual. 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.
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. 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.
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 a 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'.
Example:
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:
This is an example of a typical pipe organ stoplist, showing both common stop names and conventional formatting (flue pipes listed in black, reed pipes listed in red):
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: