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 wider interval.