In modern Western tonal music theory an augmented unison or augmented prime is the interval between two notes on the same staff position, or having 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. 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.
Diminished unison 
The augmented unison is occasionally referred to as a diminished unison, especially when descending. Some sources consider this term to be a theoretic necessity with regard to the inversion of the augmented octave, instead of its augmented counterpart commonly proposed as such. The first author to employ the 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. However, this argument runs counter to the fact that any of the greater intervals can be diminished below the unison, if only it is diminished enough times. The second for example (see diminished second) needs only one diminution to become less than 1:1 in ratio - at least in pythagorean tuning, yielding an interval essentially descending, or 'negative', in character. Thirds, fourths and so on need multiple diminution to be lowered beyond the unison. There seems to be no reason to except the unison itself, as an interval, from this diminishing principle. Nevertheless, the unison is unique in that here the qualities augmented and diminished are interchangeable with regard to the actual proportion of the interval. The distinction between the two appears to arise not from essence but solely from context and point of reference.
See also 
- ^ a b Porter, Steven (1986). Music, A Comprehensive Introduction, p.66. ISBN 978-0-935016-81-9.
- ^ Burrows, Terry (1999). How To Read Music, p.62. ISBN 978-0-312-24159-9.
- ^ Blood, Brian (2008 rev 2009). "Intervals". Music theory online. Dolmetsch Musical Instruments. Retrieved 25 December 2009.
- ^ Rushton, Julian. "Unison (prime)]". Grove Music Online. Oxford Music Online. Retrieved August 2011. (subscription needed)
- ^ 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."