Set (music)
A set (pitch set, pitchclass set, set class, set form, set genus, pitch collection) in music theory, as in mathematics and general parlance, is a collection of objects. In musical contexts the term is traditionally applied most often to collections of pitches or pitchclasses, but theorists have extended its use to other types of musical entities, so that one may speak of sets of durations or timbres, for example.^{[2]}
A set by itself does not necessarily possess any additional structure, such as an ordering. Nevertheless, it is often musically important to consider sets that are equipped with an order relation (called segments); in such contexts, bare sets are often referred to as "unordered", for the sake of emphasis.^{[4]}
Twoelement sets are called dyads, threeelement sets trichords (occasionally "triads", though this is easily confused with the traditional meaning of the word triad). Sets of higher cardinalities are called tetrachords (or tetrads), pentachords (or pentads), hexachords (or hexads), heptachords (heptads or, sometimes, mixing Latin and Greek roots, "septachords"—e.g.,^{[5]}), octachords (octads), nonachords (nonads), decachords (decads), undecachords, and, finally, the dodecachord.
A timepoint set is a duration set where the distance in time units between attack points, or timepoints, is the distance in semitones between pitch classes.^{[6]}
Serial
In the theory of serial music, however, some authors (notably Milton Babbitt^{[7]}) use the term "set" where others would use "row" or "series", namely to denote an ordered collection (such as a twelvetone row) used to structure a work. These authors speak of "twelve tone sets", "timepoint sets", "derived sets", etc. (See below.) This is a different usage of the term "set" from that described above (and referred to in the term "set theory").
For these authors, a set form (or row form) is a particular arrangement of such an ordered set: the prime form (original order), inverse (upside down), retrograde (backwards), and retrograde inverse (backwards and upside down).^{[2]}
A derived set is one which is generated or derived from consistent operations on a subset, for example Webern's Concerto, Op.24, in which the last three subsets are derived from the first:^{[8]}
B B♭ D E♭ G F♯ G♯ E F C C♯ A
Represented numerically as the integers 0 to 11:
0 11 3 4 8 7 9 5 6 1 2 10
The first subset (B B♭ D) being:
0 11 3 primeform, intervalstring = <1 +4>
The second subset (E♭ G F♯) being the retrogradeinverse of the first, transposed up one semitone:
3 11 0 retrograde, intervalstring = <4 +1> mod 12 3 7 6 inverse, intervalstring = <+4 1> mod 12 + 1 1 1  = 4 8 7
The third subset (G♯ E F) being the retrograde of the first, transposed up (or down) six semitones:
3 11 0 retrograde + 6 6 6  9 5 6
And the fourth subset (C C♯ A) being the inverse of the first, transposed up one semitone:
0 11 3 prime form, intervalvector = <1 +4> mod 12 0 1 9 inverse, intervalstring = <+1 4> mod 12 + 1 1 1  1 2 10
Each of the four trichords (3note sets) thus displays a relationship which can be made obvious by any of the four serial row operations, and thus creates certain invariances. These invariances in serial music are analogous to the use of commontones and commonchords in tonal music.
Nonserial
The fundamental concept of a nonserial set is that it is an unordered collection of pitch classes.^{[10]}
The normal form of a set is the most compact ordering of the pitches in a set.^{[11]} Tomlin defines the "most compact" ordering as the one where, "the largest of the intervals between any two consecutive pitches is between the first and last pitch listed".^{[11]} For example, the set (0,2) (a major second) is in normal form while the set (0,10) (a minor seventh, the inversion of a major second) is not, its normal form being (10,0).
Rather than the "original" (untransposed, uninverted) form of the set the prime form may be considered either the normal form of the set or the normal form of its inversion, whichever is more tightly packed.^{[12]} Forte (1973) and Rahn (1980) both list the prime forms of a set as the most leftpacked possible version of the set. Forte packs from the left and Rahn packs from the right ("making the small numbers smaller," versus making, "the larger numbers ... smaller"^{[13]}). However, these only differ in five instances^{[13]} and are the result of different algorithms (Rahn's being preferred by programmers).^{[14]}
Vectors
See also
Further reading
 Schuijer, Michiel (2008). Analyzing Atonal Music: PitchClass Set Theory and Its Contexts. ISBN 9781580462709.
References
 ^ Whittall, Arnold (2008). The Cambridge Introduction to Serialism, p.165. New York: Cambridge University Press. ISBN 9780521682008 (pbk).
 ^ ^{a} ^{b} Wittlich, Gary (1975). "Sets and Ordering Procedures in TwentiethCentury Music", Aspects of TwentiethCentury Music, p.475. Wittlich, Gary (ed.). Englewood Cliffs, New Jersey: PrenticeHall. ISBN 0130493465.
 ^ Whittall (2008), p.127.
 ^ Morris, Robert (1987). Composition With PitchClasses: A Theory of Compositional Design, p.27. Yale University Press. ISBN 0300036841.
 ^ Rahn (1980), 140.
 ^ Wittlich (1975), p.476.
 ^ See any of his writings on the twelvetone system, virtually all of which are reprinted in The Collected Essays of Milton Babbitt, S. Peles et. al, eds. Princeton University Press, 2003. ISBN 0691089663.
 ^ Wittlich (1975), p.474.
 ^ Whittall (2008), p.97.
 ^ John Rahn, Basic Atonal Theory (New York: Longman; London and Toronto: Prentice Hall International, 1980), pp.27–28. ISBN 0582281172 (Longman); ISBN 0028731603 (Prentice Hall International). Reprinted 1987 (New York: Schirmer Books; London: Collier Macmillan, 1980), p.27. ISBN 0028731603.
 ^ ^{a} ^{b} Tomlin, Jay. "All About Set Theory: What is Normal Form?", JayTomlin.com.
 ^ Tomlin, Jay. "All About Set Theory: What is Prime Form?", JayTomlin.com.
 ^ ^{a} ^{b} Nelson, Paul (2004). "Two Algorithms for Computing the Prime Form", ComposerTools.com.
 ^ Tsao, Ming (2007). Abstract Musical Intervals: Group Theory for Composition and Analysis, p.99, n.32. ISBN 9781430308355. Algorithms given in Morris, Robert (1991). Class Notes for Atonal Music Theory, p.103. Frog Peak Music.
External links
 "Set Theory Calculator", JayTomlin.com. Calculates normal form, prime form, Forte number, and interval class vector for a given set and vice versa.

