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"). Some stops are tuned to notes "in-between" the octaves and are called "Mutations" (see below). 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 than 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 128′, 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.
Like the unison and octave stops, the length label of a mutation stop indicates 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. That is, with a 22⁄3' stop drawn, pressing middle C sounds the G that is the 12th diatonic note above.
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).