Pitch class

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A pitch class is a set (equivalence class) of all pitches that are a whole number of octaves (2/1) apart, e.g., the pitch class C consists of the Cs in all octaves. Thus the pitch class "C" is the set

[math]{..., C_{-2}, C_{-1}, C_0, C_1, C_2 ...}[/math]

In terms of frequencies expressed in hertz, assuming a base frequency for middle C of 262 Hz, this would be {... 65.5, 131, 262, 524, 1028 ...}. In terms of midi note numbers, we can write it as {... 36, 48, 60, 72, 84 ...}.

In a nonoctave xenharmonic system, an interval other than the octave might be used to define equivalence. For example, in Bohlen-Pierce tuning and other equal divisions per tritave, all pitches separated by a whole number of tritaves (3/1) may be considered equivalent.