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; for example, the pitch class C consists of the Cs in all octaves. "The pitch class C stands for all possible Cs, in whatever octave position." Important to musical set theory, a pitch class is "all pitches related to each other by octave, enharmonic equivalence, or both". Thus, using scientific pitch notation, the pitch class C is the set

$${\displaystyle \{\mathrm {C} _{n}:n{\text{ is an integer}}\}={ m {\{\cdots ,C_{-2},C_{-1},C_{0},C_{1},C_{2},\cdots \}}}}$$

Although there is no formal upper or lower limit to this sequence, only a few of these pitches are audible to humans. 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 (as opposed to tone height), just as 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.