They are notated as follows:
- PI(a,b) = b - a
- PI(0,3) = 3 - 0
- PI(0,15) = 15 - 0
The PIC is notated and related to the PI thus:
- PIC(0,15) = PI(0,15) mod 12 = (15 - 0) mod 12 = 15 mod 12 = 3
the other way.
One can also measure the distance between two pitches without taking into account direction with the unordered pitch interval, similar to the interval of tonal theory. This may be defined as:
The interval between pitch classes may be measured with ordered and unordered pitch class intervals. The ordered one, also called directed interval, may be considered the measure upwards, which, since we are dealing with pitch classes, depends on whichever pitch is chosen as 0. Thus the ordered pitch class interval, i<x, y>, may be defined as:
- (in modular 12 arithmetic)
Ascending intervals are indicated by a positive value, and descending intervals by a negative one.
- Schuijer, Michiel (2008). Analyzing Atonal Music: Pitch-Class Set Theory and Its Contexts, Eastman Studies in Music 60 (Rochester, NY: University of Rochester Press, 2008), p. 35. ISBN 978-1-58046-270-9.
- Schuijer (2008), p.36.
- John Rahn, Basic Atonal Theory (New York: Longman, 1980), 21.
- John Rahn, Basic Atonal Theory (New York: Longman, 1980), 22.