A tetrad is a set of four notes in music theory. When these four notes form a tertian chord they are more specifically called 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 20th- and 21st-century music, however, where they are more generally referred to as tetrads (see, for example, , , and ). Allen Forte in his The Structure of Atonal Music never uses the term "tetrad", but occasionally employs the word tetrachord to mean any collection of four pitch classes . In 20th-century music theory, such sets of four pitch classes are usually called "tetrachords" (; ).