In musical set theory, an interval class (often abbreviated: ic), also known as unordered pitchclass interval, interval distance, undirected interval, or (completely incorrectly, since this would mean, e.g., that a perfect fourth and a minor second are inversionally equivalent and belong to the same interval class, as would a unison and a tritone) interval mod 6 (Rahn 1980, 29; Whittall 2008, 273–74), is the shortest distance in pitch class space between two unordered pitch classes. For example, the interval class between pitch classes 4 and 9 is 5 because 9 − 4 = 5 is less than 4 − 9 = −5 ≡ 7 (mod 12). See modular arithmetic for more on modulo 12. The largest interval class is 6 since any greater interval n may be reduced to 12 − n.
Use of interval classes
The concept of interval class was created to account for octave, enharmonic, and inversional equivalency.^{[original research?]} Consider, for instance, the following passage:
(To hear a MIDI realization, click the following: 106 KB (help·info)
In the example above, all four labeled pitchpairs, or dyads, share a common "intervallic color." In atonal theory, this similarity is denoted by interval class—ic 5, in this case. Tonal theory, however, classifies the four intervals differently: interval 1 as perfect fifth; 2, perfect twelfth; 3, diminished sixth; and 4, perfect fourth. Thus we see that in a dodecaphonic (i.e., chromatic) context, terminology tailored for the analysis of heptatonic (i.e., diatonic) music is often no longer suitable.^{[original research?]}
Incidentally, the example's pitch collection forms an octatonic set.^{[original research?]}
Notation of interval classes
The unordered pitch class interval i (a, b) may be defined as
where i <a, b> is an ordered pitch class interval (Rahn 1980, 28).
While notating unordered intervals with parentheses, as in the example directly above, is perhaps the standard, some theorists, including Robert Morris (1991), prefer to use braces, as in i {a,b}. Both notations are considered acceptable.
Table of interval class equivalencies
Interval Class Table
ic 
included intervals 
tonal counterparts 
0 
0 
unison and octave 
1 
1 and 11 
minor 2nd and major 7th 
2 
2 and 10 
major 2nd and minor 7th 
3 
3 and 9 
minor 3rd and major 6th 
4 
4 and 8 
major 3rd and minor 6th 
5 
5 and 7 
perfect 4th and perfect 5th 
6 
6 
augmented 4th and diminished 5th 
Sources
 Morris, Robert (1991). Class Notes for Atonal Music Theory. Hanover, NH: Frog Peak Music.
 Rahn, John (1980). Basic Atonal Theory. ISBN 0028731603. For forumala definitions only.
 Whittall, Arnold (2008). The Cambridge Introduction to Serialism. New York: Cambridge University Press. ISBN 9780521682008 (pbk).
Further reading
External links
