Trichord

In late-19th to early 20th-century Russian musicology, the term trichord (трихорд (/trixоrd/)) meant something more specific: a set of three pitches, each at least a tone apart but all within the range of a fourth or fifth.

V. Gippius, A. V. Rudnyova, N. M. Bachinskaya, L. S. Mukharinskaya, among others) boycotted the use of the term altogether, yet it could still be seen in the mid-20th century due to its heavy use in the works of earlier theorists.

Unlike the tetrachord and hexachord, there is no traditional standard scale arrangement of three notes, nor is the trichord necessarily thought of as a harmonic entity.

[5] 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.

A computer program can quickly iterate all the triads and remove the ones that are merely transpositions of others, leaving (as noted above) nineteen or, to within inversional equivalence, twelve.