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An organ stop is any of the characteristic sounds (timbres) available on a pipe organ. A particular stop is usually selected by pulling a knob (also called a stop) by the side of the keyboard.
Stops have particular names according to the kind of pipes used to create the sound.
Single-rank stops, the majority, are usually marked with an arabic numeral indicating the pitch; it originated as the length, in feet, of the rank's longest pipe. Registration usually begins with an 8' on the manual and a 16' on the pedal; the pedal thus normally sounds an octave lower than played, providing the "full-bodied" sound typical of the organ. Halving the number raises the pitch by an octave; in growing a registration it is usual to draw next the 4', 2', etc. on the manual and the 8', 4', etc. on the pedal. Numbers other than powers of 2 appear; these are called mutants. The 5 1/3', 2 2/3', 1 1/3' series sounds a G for a C; the rarer 3 1/5', 1 3/5', 4/5' sounds an E for a C.
The most common multi-rank stops are mixtures. They're usually marked with a roman numeral indicating the number of ranks. Though it seems somehow wasteful to not provide the ranks separately, a mixture rank may break - drop an octave - as one goes up the scale to avoid shrillness, making it inappropriate for use apart. The name applied to the mixture usually codes for the particular chord provided, but octaves and fifths are most common; e.g. a Quint III might provide the 5th, 8ve, and 12th; an Acuta IV might have the 15th, 17th, 19th, and 23rd. Mixtures are usually used to boost harmonics, but some are appropriate as solo stops.
The next most common multi-rank stops are celestes, two identical ranks except one is slightly out of tune. But celestes are often presented as pairs of stops, since they don't break like mixtures.
Sometimes a single rank has multiple stops; this is called borrowing or unification. The stops may draw the rank to different keyboards or at different pitches.
Flue stops, such as principals, flutes, and strings, are essentially end-blown whistles. Reed stops, such as trumpets or vox humanas consist of a metal reed vibrating in a full- or partial-length resonator. Short-length reed resonators of unusual shape are often called "vowel chambers."
Names of stops are somewhat organ builder specific - a stop with a same name can have slightly different timbres on different organs.
In pipe organs, each key on a keyboard is connected to each pipe in the ranks or rows of pipes connected to that particular keyboard. When the key is pressed, air is allowed to flow up from the blower.
In order to control what pipes will sound, stops were developed for organs. A stop is made up of material that fits underneath the pipes for each row, or rows in mixture stop. When the stop is closed, the air is kept from flowing up into the pipes in that rank. Pipes in that state are referred to as stopped. When the stop knob is pulled out, the material slides out from under the pipe. Pipes in that state are referred to as open. When a key in that rank is pushed, the air from the blower is allowed to flow up, and into the pipe to create the noise. If more than one rank is open, the air will flow into the pipes in each open rank. This gives the organist a great deal of flexibility in what sound the organ will produce.
Organ consoles are connected to the stops by varying means. Those organs that have a console where the stop knobs are directly connected to the stops are referred to as having mechanical action. Those that are not directly connected often rely on electronics to control the opening and closing of a stop.
In electronic organs that do not have pipes, opening a stop merely adds the "rank" in question through various means. The means used have changed over the years since the introduction of the electric organ in the first part of the 20th century.
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