Interval class

In musical set theory, an interval class (often abbreviated: ic), also known as unordered pitch-class interval, interval distance, undirected interval, or "(even completely incorrectly) as '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.

Interval class Play .

Use of interval classes

The concept of interval class accounts for octave, enharmonic, and inversional equivalency. Consider, for instance, the following passage:

(To hear a MIDI realization, click the following: 106 KB 

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.

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
icincluded intervalstonal counterpartsextended intervals
0 0unison and octavediminished 2nd and augmented 7th
1 1 and 11minor 2nd and major 7thaugmented unison and diminished octave
2 2 and 10major 2nd and minor 7thdiminished 3rd and augmented 6th
3 3 and 9minor 3rd and major 6thaugmented 2nd and diminished 7th
4 4 and 8major 3rd and minor 6thdiminished 4th and augmented 5th
5 5 and 7perfect 4th and perfect 5thaugmented 3rd and diminished 6th
6 6augmented 4th and diminished 5th

See also

Sources
  • Morris, Robert (1991). Class Notes for Atonal Music Theory. Hanover, NH: Frog Peak Music.
  • Rahn, John (1980). Basic Atonal Theory. ISBN 0-02-873160-3.
  • Whittall, Arnold (2008). The Cambridge Introduction to Serialism. New York: Cambridge University Press. ISBN 978-0-521-68200-8 (pbk).

Further reading
  • Friedmann, Michael (1990). Ear Training for Twentieth-Century Music. New Haven: Yale University Press. ISBN 0-300-04536-0 (cloth) ISBN 0-300-04537-9 (pbk)
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