Music Structures

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Depending on the context, a trichord is either a contiguous segment of a musical scale or of a twelve-tone row, or (in accord with Allen Forte's redefinition of the term) a musical triad, that is, any three-note pitch collection.

Just as a diatonic scale is conventionally said to be constructed of two disjunct tetrachords (CDEF+GABC=CDEFGABC), a pentatonic scale can be constructed of two disjunct trichords (ACD+EGA=ACDEGA; GAC+DEG=GACDEG).

Milton Babbitt's serial theory of combinatoriality makes much of the properties of three-note, four-note, and six-note segments of a twelve-tone row, which he calls, respectively, trichords, tetrachords, and hexachords, extending the traditional sense of the terms and retaining their implication of contiguity.

Allen Forte in his The Structure of Atonal Music redefines the term trichord to mean what other theorists (notably including Howard Hanson in his Harmonic Materials of Modern Music: Resources of the Tempered Scale and Carlton Gamer in his "Some Combinational Resources of Equal-Tempered Systems") mean by the term triad, a three-note pitch collection which is not necessarily a contiguous segment of a scale or a tone row and not necessarily (in twentieth-century music) tertian or diatonic either.