Augmented unison



In modern Western tonal music theory an augmented unison or augmented prime is the interval between two notes on the same staff position, or denoted by the same note letter, whose alterations cause them, in ordinary equal temperament, to be one semitone apart. In other words, it is a unison where one note has been altered by a half-step, such as B♭ and B♮ or C♮ and C♯. The interval is often described as a chromatic semitone. The term, in its French form unisson superflu, appears to have been coined by Jean-Philippe Rameau in 1722, who also called this interval a minor semitone (semiton mineur). Historically, this interval, like the tritone, is described as being "mi contra fa", and therefore is the "diabolus in musica" (the Devil in music). In 12-tone equal temperament, it is the enharmonic equivalent of a diatonic semitone or minor second, although in other tunings the diatonic semitone is a different interval.

Diminished unison
The term diminished unison or diminished prime is also found occasionally. It is found once in Rameau's writings, for example, as well as subsequent French, German, and English sources. Other sources reject the possibility or utility of the diminished unison on the grounds that any alteration to the unison increases its size, thus augmenting rather than diminishing it. The term is sometimes justified as a negative-numbered interval, and also in terms of violin double-stopping technique on analogy to parallel intervals found on other strings. Some theoreticians make a distinction for this diminished form of the unison, stating it is only valid as a melodic interval, not a harmonic one.
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 * Stefan Kostka and Dorothy Payne (2004). Tonal Harmony (Boston: McGraw-Hill): 21. ISBN 978-0-07-285260-8. "There is no such thing as a diminished unison."
 * Michael Pilhofer and Holly Day (2006). Music Theory for Dummies (Hoboken, NJ: John Wiley & Sons, Inc.): 113. ISBN 978-0-7645-7838-0. "There is no such thing as a diminished unison, because no matter how you change the unisons with accidentals, you are adding half steps to the total interval."
 * Andrew Surmani, Karen Farnum Surmani, and Morton Manus (2009). Alfred's Essentials of Music Theory: A Complete Self-Study Course for All Musicians ([s.l.]: Alfred Music Publishing): 135. ISBN 0-7390-3635-1. "Since lowering either note of a perfect unison would actually increase its size, the perfect unison cannot be diminished, only augmented."
 * W. S. B. Mathews (1909). "Editorial: Prof. White's Harmony and Ear-Training", The Journal of School Music 1, no. 9 (June): 260–63. Citation on 263: "What he [Prof. White in Harmony and Ear Training] calls the 'diminished prime or unison' cannot possibly occur. It is simply an augmented unison. Because unison is 'the relation of two tones at the same pitch,' and when one of these is chromatically distanced, it creates the contradiction in terms known as 'augmented' unison; but the other term, 'diminished unison' is impossible on the face of it, because the 'same pitch' cannot be made less."
 * Smith, Uselma Clarke (1916). Keyboard Harmony, p.15. The Boston Music Company. "Note that the diminished unison and octave are not commonly used."
 * Jim Aikin (2004). A Player's Guide to Chords & Harmony (San Francisco: Backbeat Books): 32. ISBN 978-0-87930-798-1. "In case you were wondering, there's no such thing as a diminished unison."
 * Arthur Foote and Walter Raymond Spalding (1905). Modern Harmony in Its Theory and Practice, p.5. Arthur P. Schmidt. "a diminished unison is unthinkable, and the diminished 2d and 9th are of no practical use:..."