 Allen Forte occasionally uses the term "tetrachord" to mean what he elsewhere calls a "tetrad" or simply a "4-element set" – a set of any four pitches or "pitch classes".In twelve-tone theory, the term may have the special sense of any consecutive four notes of a twelve-tone row.Tetrachords based upon equal-tempered tuning were also used to approximate common heptatonic scales in use in Indian, Hungarian, Arabian and Greek musics.