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Registration is the technique of choosing and combining the stops of a pipe organ in order to produce a particular sound. Registration can also refer to a particular combination of stops. The registration chosen for a particular piece will be determined by a number of factors, including the composer's indications (if any are given), the time and place in which the piece was composed, the organ the piece is played upon, and the acoustic in which the organ resides.
Pitch and timbre
The pitch produced by a pipe is a function of its length. An organ stop may be tuned to sound (or "speak at") the pitch normally associated with the key that is pressed (the "unison pitch"), or it may speak at a fixed interval above or below this pitch (an "octave pitch"). The pitch of a rank of pipes is denoted by a number on the stop knob. A stop that speaks at unison pitch (the "native pitch" for that note; the pitch you would hear if you pressed that same key on a piano) is known as an 8' (pronounced "eight foot") stop. This nomenclature refers to the approximate length of the longest pipe in that rank.
The octave sounded by a given pipe is inversely exponentially proportional to its length ("1/2 the length = double the pitch"), meaning that a 4' stop speaks exactly one octave higher an 8' stop. Likewise, a 2' stop speaks exactly one octave higher than a 4' stop. Conversely, a 16' stop speaks exactly one octave below an 8' stop; and a 32′ stop speaks exactly one octave below a 16' stop. Lengths used in actual organs include 64', 32', 16', 8', 4', 2', 1', and 1/2'.
Ranks that do not speak at a unison or octave pitch, but rather at a non-octave interval to the unison pitch, are called mutation stops (or, simply, "mutations"). Because they sound at intervals other than an octave (2:1 ratio) above or below the unison sound, they are rarely used on their own; rather, they are combined with unison stops to create different tone colors.
The length label of a mutation stop gives the answer as to what pitch the rank sounds. For example, a stop labeled 22⁄3' sounds at the interval of a twelfth (one octave plus a fifth; or 3:1 ratio) above unison pitch.
Mutations usually sound at pitches in the harmonic series of the fundamental. In some large organs, non harmonic mutations are used infrequently, as there is more "fundamental" to support. Such mutations that sound at the fifth just above or below the fundamental, can create the impression of a stop an octave lower than the fundamental, especially when low frequencies are involved. Whenever possible or practical, mutations are tuned a mean tone interval away from the fundamental.
Certain stops called mixtures contain multiple ranks of pipes sounding at consecutive octaves and fifths (and in some cases, thirds) above unison pitch. The number of ranks in a mixture is denoted by a Roman numeral on the stop knob; for example, a stop labeled "Mixture V" would contain five pipes for every note. So for every key pressed, five different pipes sound (all controlled by the same stop).
National styles of registration
In the seventeenth century, national styles of organ building began to emerge. Organs had certain unique characteristics that were common to organs in the country in which they were built. Registration techniques developed that mirrored the characteristics in the organs of each national style.