In Western classical music, an augmented second is an interval created by widening a major second by a chromatic semitone, spanning three semitones and enharmonically equivalent to a minor third in 12-tone equal temperament. For instance, the interval from C to D is a major second, two semitones wide, and the interval from C to D♯ is an augmented second, spanning three semitones.
In classical music from Western culture, a diminished third () is the musical interval produced by narrowing a minor third by a chromatic semitone. For instance, the interval from A to C is a minor third, three semitones wide, and both the intervals from A♯ to C, and from A to C♠are diminished thirds, two semitones wide. Being diminished, it is considered a dissonant interval.
In 12-tone equal temperament a diminished third is enharmonic with the major second, both having a value of 200 cents. However, in meantone tunings with fifths flatter than 700 cents, the diminished third is wider than the major second. In 19 equal temperament it is in fact enharmonically equivalent to an augmented second, both having a value of 252.6 cents. In 31 equal temperament it has a more typical value of 232.3 cents. In a twelve-note keyboard tuned in a meantone tuning from Eâ™ to G♯, the dimininished third appears between C♯ and Eâ™, and again between G♯ and Bâ™.
In Western music theory, an augmented unison, augmented prime, or chromatic semitone is the interval between two notes on the same staff position (same letter) that differ by one chromatic alteration. For example, the intervals between B♠and B♮, as well as C♮ and C♯, are augmented unisons.
In 12-tone equal temperament, the augmented unison is rendered by the semitone; in this case it is enharmonically equivalent to the minor second.