Mutation stop

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 "" 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, 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, 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.

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Wikipedia