In modern Western tonalmusic 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. 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 wider interval.
The augmented unison is occasionally referred to also as a diminished unison. The first author to employ this term was apparently William White, in 1907. Many 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.
^White, William Alfred (1907). Harmony and ear-training. New York, Boston [etc.]: Silver, Burdett & Company.
^Kostka and Payne (2003). Tonal Harmony, p.21. ISBN 0-07-285260-7. "There is no such thing as a diminished unison."
^Day and Pilhofer (2007). Music Theory for Dummies, p.113. ISBN 0-7645-7838-3. "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."
^Surmani, Andrew; Karen Farnum Surmani, Morton Manus (2009) Alfred's Essentials of Music Theory: A Complete Self-Study Course for All Musicians Alfred Music Publishing. ISBN 0-7390-3635-1, p. 135 "Since lowering either note of a perfect unison would actually increase its size, the perfect unison cannot be diminished, only augmented."
^(1908). The Journal of School Music, p.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."
^Gardner, Carl Edward (1912). Essentials of Music Theory, p.38. C. Fischer. ISBN 978-1-4400-6780-8. "The prime is also called an unison, but in speaking of intervals, it should always be called a prime. Correctly speaking, a perfect prime is not an interval, but in the theory of music it is so called. There is good reason for making this error, but none for called a diminished prime a diminished unison."
^Smith, Uselma Clarke (1916). Keyboard Harmony, p.15. The Boston Music Company. "Note that the diminished unison and octave are not commonly used."
^Aikin, Jim (2004). A Player's Guide to Chords & Harmony, p.32. ISBN 978-0-87930-798-1. "In case you were wondering, there's no such thing as a diminished unison."
^Arthur Foote, 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:..."