Tetrad (music)

In music theory, a tetrad is a set of four notes. When these four notes form a tertian chord they are more particularly and more commonly referred to as a seventh chord, after the diatonic interval from the root of the chord to its fourth note (in root position close voicing). Four-note chords are often formed of intervals other than thirds in twentieth-century music, however, where they are more generally referred to as tetrads (See, for example, Howard Hanson's Harmonic Materials of Modern Music: Resources of the Tempered Scale). A four-note segment of a scale or twelve-tone row is more particularly known as a tetrachord, although Allen Forte in his The Structure of Atonal Music uses the term tetrachord as if it were synonymous with tetrad.