From Wikipedia, the free encyclopedia

In musical set theory, an interval class, or ic, 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. See modular arithmetic for more on modulo 12.

Contents 1 Use of interval classes 2 Notation of interval classes 3 Table of interval class equivalencies 4 Sources 5 Further reading 6 External link

The concept of interval class was created to account for octave, enharmonic, and inversional equivalency. 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 pitch-pairs, 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.

Incidentally, the example's pitch collection forms an octatonic set.

The largest interval class is 6 since all larger intervals may be reduced.

The unordered pitch class interval i (a,b) may be defined as:

i(a,b) = the smaller of i < a,b > and i < b,a > , where i <a,b> is an ordered pitch class interval.

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.

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

• Morris, Robert (1991). Class Notes for Atonal Music Theory. ASIN B0006DHW9I.
• Rahn, John (1980). Basic Atonal Theory. ISBN 0-02-873160-3. For forumala definitions only.

• Friedmann, Michael (1990). Ear Training for Twentieth Century Music. ISBN 0-300-04537-9.

• Solomon's Set Theory Primer