Permutation (music)

In musical set theory, a permutation (order) of a set is any ordering of the elements of that set. A specific arrangement of a set of discrete entities, or parameters, such as pitch, dynamics, or timbre. Different permutations may be related by transformation, through the application of zero or more operations, such as transposition, inversion, retrogradation, circular permutation (also called rotation), or multiplicative operations (such as the cycle of fourths and cycle of fifths transforms). These may produce reorderings of the members of the set, or may simply map the set onto itself. Order is particularly important in the theories of composition techniques originating in the 20th century such as the twelve-tone technique and serialism. Analytical techniques such as set theory take care to distinguish between ordered and unordered collections. In traditional theory concepts like voicing and form include ordering; for example, many musical forms, such as rondo, are defined by the order of their sections. The permutations resulting from applying the inversion or retrograde operations are categorized as the prime form's inversions and retrogrades, respectively. Applying both inversion and retrograde to a prime form produces its retrograde-inversions, considered a distinct type of permutation. Permutation may be applied to smaller sets as well. However, transformation operations of such smaller sets do not necessarily result in permutation the original set. Here is an example of non-permutation of trichords, using retrogradation, inversion, and retrograde-inversion, combined in each case with transposition, as found within the tone row (or twelve-tone series) from Anton Webern's Concerto:

If the first three notes are regarded as the "original" cell, then the next 3 are its transposed retrograde-inversion (backwards and upside down), the next three are the transposed retrograde (backwards), and the last 3 are its transposed inversion (upside down). Not all prime series have the same number of variations because the transposed and inverse transformations of a tone row may be identical, a quite rare phenomenon: less than 0.06% of all series admit 24 forms instead of 48. One technique facilitating twelve-tone permutation is the use of number values corresponding with musical letters. The first note of the first of the primes, actually prime zero (commonly mistaken for prime one), is represented by 0. The rest of the numbers are counted half-step-wise such that:

B = 0 F = 6 C = 1 F ♯ / G ♭ = 7 C ♯ / D ♭ = 2 G = 8 D = 3 G ♯ / A ♭ = 9 D ♯ / E ♭ = 4 A = 10 E = 5 A ♯ / B ♭ = 11 {\displaystyle {\begin{aligned}{\rm {B}}&=0&{\rm {F}}&=6\\{\rm {C}}&=1&{\rm {F\sharp /G\flat }}&=7\\{\rm {C\sharp /D\flat }}&=2&{\rm {G}}&=8\\{\rm {D}}&=3&{\rm {G\sharp /A\flat }}&=9\\{\rm {D\sharp /E\flat }}&=4&{\rm {A}}&=10\\{\rm {E}}&=5&{\rm {A\sharp /B\flat }}&=11\end{aligned}}}

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