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(advanced music theory)

The term "interval class" refers to a mapping of all intervals onto six distinct classes, distinguished by the number of semitones in each interval class.

A single interval class contains all intervals that can be arranged through octave transposition and enharmonic respelling to contain the same number of semitones. Thus a major third and a minor sixth are in the same interval class; also in this interval class would be the major tenth, compound third, diminished fourth, and so on.

Under octave transposition and enharmonic respelling, there are only six distinct interval classes, each containing a different number of semitones, from one semitone, up to six. Thus the names: IC 1, IC 2,... IC 6. A basic mapping:

m2, M7 --> IC 1
M2, m7 --> IC 2
m3, M6 --> IC 3
M3, m6 --> IC 4
P4, P5 --> IC 5
-5, +4 --> IC 6 (the tritone)
