A tetrad is a set of four notes in music theory

Music theory

Music theory is the study of how music works. It examines the language and notation of music. It seeks to identify patterns and structures in composers' techniques across or within genres, styles, or historical periods...

. When these four notes form a tertian

Tertian

In music theory, tertian describes any piece, chord, counterpoint etc. constructed from the interval of a third...

 chord they are more specifically called a seventh chord

Seventh chord

A seventh chord is a chord consisting of a triad plus a note forming an interval of a seventh above the chord's root. When not otherwise specified, a "seventh chord" usually means a major triad with an added minor seventh...

, after the diatonic

Diatonic and chromatic

Diatonic and chromatic are terms in music theory that are most often used to characterize scales, and are also applied to intervals, chords, notes, musical styles, and kinds of harmony...

 interval

Interval (music)

In music theory, an interval is a combination of two notes, or the ratio between their frequencies. Two-note combinations are also called dyads...

 from the root

Root (chord)

In music theory, the root of a chord is the note or pitch upon which a triadic chord is built. For example, the root of the major triad C-E-G is C....

 of the chord

Chord (music)

A chord in music is any harmonic set of two–three or more notes that is heard as if sounding simultaneously. These need not actually be played together: arpeggios and broken chords may for many practical and theoretical purposes be understood as chords...

 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, Hanson

Howard Hanson

Howard Harold Hanson was an American composer, conductor, educator, music theorist, and champion of American classical music. As director for 40 years of the Eastman School of Music, he built a high-quality school and provided opportunities for commissioning and performing American music...

 1960, , Gamer 1967, 37 & 52, and Forte 1985, 48–51, 53). 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 class

Pitch class

In music, a pitch class is a set of all pitches that are a whole number of octaves apart, e.g., the pitch class C consists of the Cs in all octaves...

es (Forte 1973, 1, 18, 68, 70, 73, 87, 88, 21, 119, 123, 124, 125, 138, 143, 171, 174, and 223). In 20th-century music theory, such sets

Set (music)

A set in music theory, as in mathematics and general parlance, is a collection of objects...

of four pitch classes are usually called "tetrachords" (Anon. 2001).