Pitch interval

In musical set theory, a pitch interval (PI or ip) is the number of semitones that separates one pitch from another, upward or downward.

They are notated as follows:

For example C4 to D♯4 Play is 3 semitones:

While C4 to D♯5 Play is 15 semitones:

However, under octave equivalence these are the same pitches (D♯4 & D♯5, Play ), thus the #Pitch-interval class may be used.