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Stops

Each stop usually controls one rank of pipes, although mixtures and undulating stops (such as the Voix céleste) control multiple ranks. The name of the stop reflects not only the stop's timbre and construction, but also the style of the organ in which it resides. For example, the names on an organ built in the north German Baroque style generally will be derived from the German language, while the names of similar stops on an organ in the French Romantic style will usually be French. Most countries tend to use only their own languages for stop nomenclature. English-speaking nations as well as Japan are more receptive to foreign nomenclature. Stop names are not standardized: two otherwise identical stops from different organs may have different names.

To facilitate a large range of timbres, organ stops exist at different pitch levels. A stop that sounds at unison pitch when a key is depressed is called an 8′ (pronounced "eight-foot") pitch. This refers to the speaking length of the lowest-sounding pipe in that rank, which is approximately eight feet (2.4 m). For the same reason, a stop that sounds an octave higher is at 4′ pitch, and one that sounds two octaves higher is at 2′ pitch. Likewise, a stop that sounds an octave lower than unison pitch is at 16′ pitch, and one that sounds two octaves lower is at 32′ pitch. Stops of different pitch levels are designed to be played simultaneously.

The label on a stop knob or rocker tab indicates the stop's name and its pitch in feet. Stops that control multiple ranks display a Roman numeral indicating the number of ranks present, instead of pitch. Thus, a stop labelled "Open Diapason 8′ " is a single-rank diapason stop sounding at 8′ pitch. A stop labelled "Mixture V" is a five-rank mixture.

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. In this case, the full rank of pipes (now an extended rank) is one octave longer than the keyboard.

Special unpitched stops also appear in some organs. Among these are the Zimbelstern (a wheel of rotating bells), the nightingale (a pipe submerged in a small pool of water, creating the sound of a bird warbling when wind is admitted), and the effet d'orage ("thunder effect", a device that sounds the lowest bass pipes simultaneously). Standard orchestral percussion instruments such as the drum, chimes, celesta, and harp have also been imitated in organ building.