Pitch class

In music, a pitch class (p.c. or pc) 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."[1] Important to musical set theory, a pitch class is "all pitches related to each other by octave, enharmonic equivalence, or both."[2] Thus, using scientific pitch notation, the pitch class "C" is the set

{Cn : n is an integer} = {..., C−2, C−1, C0, C1, C2, C3 ...}.
Perfect octave Play 
All Cs from C1 to C7 inclusivePlay .

Although there is no formal upper or lower limit to this sequence, only a few of these pitches are 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".[3] A chroma is an attribute of pitches (as opposed to tone height), just like hue is an attribute of color. A pitch class is a set of all pitches that share the same chroma, just like "the set of all white things" is the collection of all white objects.[4]

Note that in standard Western equal temperament, distinct spellings can refer to the same sounding object: B3, 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.