Trichord

Trichord is a term in music theory used to describe a group of three pitch classes, considered modulo octave. In the context of pitch-class set theory, a trichord is any unordered set of three distinct pitch classes; it serves as a basic building block for describing musical materials regardless of register or voicing. The concept emphasizes the interval structure among the three notes rather than their exact pitch names. A trichord can form a traditional triad, such as the major triad C–E–G (pitch-class set {0,4,7}) or the minor triad C–E♭–G ({0,3,7}); but it also includes many other three-note combinations that do not fit conventional tertian harmony, including clusters and dispersed triplets.

The study of trichords often involves operations such as transposition (shifting all pitch classes by the same amount modulo 12) and inversion (reflecting around a given axis), and the resulting equivalence classes are called trichord set classes. In twelve-tone and post-tonal analysis, each trichord can be described by its interval content or by its prime form. There are 220 possible trichords in a chromatic (12-tone) system, and far fewer distinct set classes when transposition and inversion are taken into account.

Applications include analysis of scales, chords, and sonorities in both tonal and atonal music, as well as in ethnomusicology where three-note sonorities are common. The term is often contrasted with “triad,” which is specifically a consonant three-note chord with typical tertian structure.