Editing Pipe organ (section)

===Stops===
{{Main|Organ stop}}

Each stop usually controls one rank of pipes, although [[Mixture (music)|mixtures]] and undulating stops (such as the [[Voix céleste]]) control multiple ranks.<ref name="Bicknell 26-27">Bicknell "Organ construction", 26–27.</ref> 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.{{Citation needed|date=February 2019}} Stop names are not standardized: two otherwise identical stops from different organs may have different names.<ref>Bicknell "Organ construction", 27–28.</ref>

To facilitate a large range of timbres, organ stops exist at different pitch levels. A stop that sounds at [[unison|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 {{convert|8|ft|spell=in}}. 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.<ref name="Bicknell 26-27"/> 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.<ref>Johnson, David N. (1973). [https://books.google.com/books?id=e447xDPoQqoC&pg=PA9 Instruction Book for Beginning Organists]. Revised edition. Augsburg Fortress. p. 9. {{ISBN|978-0-8066-0423-7}}. Google Book search. Retrieved on 15 August 2008.</ref> Thus, a stop labelled "Open Diapason 8′ " is a single-rank [[Open diapason|diapason]] stop sounding at 8′ pitch. A stop labelled "Mixture&nbsp;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′){{efn|name=Helmholtz|This article uses the [[Helmholtz pitch notation]] to indicate specific pitches.}} 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.{{efn|The purpose of extended ranks and of their being borrowed is to save on the number of pipes. For example, without unification, three stops may use 183 pipes. With unification three stops may borrow one extended rank of 85 pipes. That's 98 fewer pipes used for those three stops.}}

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),<ref>Randel "Rossignol", 718.</ref> and the ''effet d'orage'' ("thunder effect", a device that sounds the lowest bass pipes simultaneously). Standard orchestral percussion instruments such as the drum, [[Tubular bell|chimes]], [[celesta]], and [[harp]] have also been imitated in organ building.<ref>Ahrens, 339; Kassel, 526–527</ref>

<gallery widths="200px" heights="200px">
File:Weingarten Basilika Gabler-Orgel Register rechts.jpg|Stop knobs of the Baroque organ in [[Organ of the Basilica of St. Martin (Weingarten)|Weingarten]], Germany
File:M.P. Möller Chapel Pipe Organ 1936.jpg|M.P. Möller three-rank chapel organ (1936)
</gallery>