Trichord

Webern's Concerto, Op. 24, tone row,^[1] composed of four trichords: P RI R I.

In music theory, a trichord (/traɪkɔːrd/) is a group of three different pitch classes found within a larger group.^[2] A trichord is a contiguous three-note set from a musical scale^[3] or a twelve-tone row.

In musical set theory there are twelve trichords given inversional equivalency, and, without inversional equivalency, nineteen trichords. These are numbered 1–12, with symmetrical trichords being unlettered and with uninverted and inverted nonsymmetrical trichords lettered A or B, respectively. They are often listed in prime form, but may exist in different voicings; different inversions at different transpositions. For example, the major chord, 3-11B (prime form: [0,4,7]), is an inversion of the minor chord, 3-11A (prime form: [0,3,7]). 3-5A and B are the Viennese trichord (prime forms: [0,1,6] and [0,5,6]).