Editing Organ stop (section)

==Mechanics==
<!-- Deleted image removed: [[Image:OrganConsole.JPG|thumbnail|300px|right|The organ console at St. Raphael's Cathedral in Dubuque, Iowa. The stop knobs on this console are placed to the left and right of the manuals.]] -->
<!-- Deleted image removed: [[Image:Estops.jpg|thumbnail|300px|right|Stop tabs on an electronic organ, located above the uppermost manual. Compare this placement to that of the stop knobs on the pipe organ console pictured below.]] -->

Organ pipes are physically organized within the organ into sets according to [[note (music)|note]] and [[timbre]]. 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 or 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, they operate 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]],<ref>Dalton, James. "Iberian organ music before 1700," in ''[[Cambridge Companions to Music|The Cambridge Companion to the Organ]]'', ed. Nicholas Thistlethwaite and Geoffrey Webber (Cambridge University Press, 1998), p. 165.</ref> 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 (music)|manual]].

Ranks which are neither divided nor extended (see below [[#Unification, borrowing and extension|Unification, borrowing and extension]]) generally contain as many pipes as there are keys on the keyboard to which they are assigned: in most cases 61 pipes for a rank assigned to a manual and 32 pipes for a rank assigned to the pedal.

===Methods of actuation===
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. Many organs originally built with mechanical actuators have been retrofitted with electric actuators.

Other common designs include the spring chest, the cone valve chest, and the Pitman chest.

===Unification, borrowing and extension===
The term ''unification'' refers to the practice of expanding the tonal resources of an organ without adding more pipes by allowing several different stops to control the same rank of pipes. For example, an 8′ Gedeckt may also be made available as a 4′ Gedeckt, either on the same or a different manual. When both of these stops are selected and a key (for example, C<sub>3</sub>) is pressed, two pipes of the same rank will sound: the pipe normally corresponding to the key played (C<sub>3</sub>), and the pipe one octave above that (C<sub>4</sub>).

''Borrowing'' or ''duplexing'' refers to one rank being made available from multiple stop knobs, often on different manuals or pedal.<ref>Shannon, John R. ''Understanding the Pipe Organ'', 2009, p. 83.</ref> ''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 by higher and/or lower stops. Unification and borrowing (duplexing) is mostly related to pipe organs with physical pipes; however, some (older) electronic organs also used unification and duplexing to expand the tonal resources of a limited number of synthesized virtual ranks.

While unification and extension increase the tonal resources and flexibility of the organ, greater care needs to be taken by the organist in registering the organ, particularly when the composition requires many notes to sound at the same time. In a non-unified organ, voices are scaled for their intended job. As an example, the octave (4′) diapason is generally of a smaller scale and softer than the corresponding 8′ diapason rank, whereas in unification they would be of the same strength due to using the same set of pipes. Straight reed choruses (16′, 8′ and 4′) have the luxury of ranks with different timbres, whereas a unified reed chorus has voices that are identical.

Playing with all stops out on a heavily unified/duplexed organ may result in chords that sound thinner or emphasize higher harmonics on some notes more than others, due to notes in different octaves using the same pipes instead of having their own. Part of an organist's training is to detect unification and duplexing and to create registrations that take them into account.<ref>Shannon, John R. ''Understanding the Pipe Organ'', 2009, Chapter 6.</ref> Nonetheless, heavy unification can create issues for visiting artists with limited practice times, or those improvising compositions.

Borrowing between manuals occurs in English organs from about 1700,{{Citation needed|date=September 2007}} but extension of pipe ranks for the purpose of borrowing at different pitches is a relatively recent development.{{Citation needed|date=September 2007}} Extension and unification are heavily used in [[theatre organ]]s to produce the maximum number of voices from a minimal number of pipes. It is still typical to see a significant amount of unification and duplexing in practice organs and small church organs. Traditionally, less use has been made of extension in large 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.<ref>{{cite book |last1=Audsley |first1=George Ashdown |title=The Art of Organ-Building |date=1905 |publisher=Dodd, Mead, and Company |location=New York |page=13}}</ref>{{Citation needed|date=September 2007}} Borrowing 16′ manual ranks for the pedal division is more widely employed because of the expense and space requirements of 16′ stops and the versatility this allows.