Pitch Class

In music, a pitch class is a set of all pitches that are a whole number of octaves apart, e.g., the pitch class C consists of the Cs in all octaves. "The pitch class C stands for all possible Cs, in whatever octave position." Thus, using scientific pitch notation, the pitch class "C" is the set

{C_n : n is an integer} = {..., C_-2, C_-1, C₀, C₁, C₂, C₃ ...};

although there is no formal limit to this sequence on either end, only a limited number of these pitches will actually be audible to the human ear. Pitch class is important because human pitch-perception is periodic: pitches belonging to the same pitch class are perceived as having a similar "quality" or "color", a property called octave equivalence.

Psychologists refer to the quality of a pitch as its "chroma". A "chroma" is an attribute of pitches, just like hue is an attribute of color. A "pitch class" is a set of all pitches sharing the same chroma, just like "the set of all white things" is the collection of all white objects.

Note that in standard Western equal temperament, distinct spellings can refer to the same sounding object: B♯₃, C₄, and D₄ all refer to the same pitch, hence share the same chroma, and therefore belong to the same pitch class; a phenomenon called enharmonic equivalence.