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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. Musicologist 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".

Citations

1. See, for example, Hanson 1960, pp. 18, 22, 28, 32, et passim; Gamer 1967, pp. 37 & 52; and Forte 1985, pp. 48–51, 53
2. Forte 1973, pp. 1, 18, 68, 70, 73, 87, 88, 21, 119, 123–125, 138, 143, 171, 174, and 223.
3. Anon. 2001.
4. Roeder 2001.

References

• Anonymous (2001). "Tetrachord". The New Grove Dictionary of Music and Musicians, second edition, edited by Stanley Sadie and John Tyrrell. London: Macmillan Publishers.
• Forte, Allen (1973). The Structure of Atonal Music. New Haven and London: Yale University Press. ISBN 0-300-01610-7 (cloth) ISBN 0-300-02120-8 (pbk).
• Forte, Allen (1985). "Pitch-Class Set Analysis Today". Music Analysis 4, nos. 1 & 2 (March–July: Special Issue: King's College London Music Analysis Conference 1984): 29–58.
• Gamer, Carlton (1967). "Some Combinational Resources of Equal-Tempered Systems". Journal of Music Theory 11, no. 1:32–59.
• Hanson, Howard (1960). Harmonic Materials of Modern Music: Resources of the Tempered Scale. New York: Appleton-Century-Crofts.
• Roeder, John (2001). "Set (ii)". The New Grove Dictionary of Music and Musicians, second edition, edited by Stanley Sadie and John Tyrrell. London: Macmillan Publishers.

v t e Chords
By form	Triad Major Minor Augmented Diminished Suspended Seventh Leading-tone Major Minor Dominant Dominant seventh flat five Diminished Half-diminished Diminished major Minor-major Augmented major Augmented minor Altered seventh Nondominant Harmonic seventh Extended Ninth Eleventh Thirteenth Upper structure Dominant 7♯9 Polychord Tone cluster Added / omitted Sixth Augmented sixth Lydian Seven six Specific Alpha Bridge Complexe sonore Dream Elektra Farben Grandmother Magic Mystic Northern lights Petrushka Psalms So What Tristan Viennese trichord General Mixed interval Secundal Tertian Quartal Synthetic chord Tetrad
By function	Diatonic Tonic Supertonic Mediant Subdominant Dominant Submediant Leading tone / Subtonic Altered Approach Borrowed Chromatic mediant Neapolitan Passing Secondary Secondary dominant Secondary leading-tone Secondary supertonic Other Common Contrast Primary triad Subsidiary Substitute
Techniques	Barre Block Chordioid Chord-scale system Guitar Open Power Slash
Other	Arpeggio Chord names and symbols List of chords Factor

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