Augmented seventh Augmented seventh In classical music from Western culture, an augmented seventh is an interval produced by widening a major seventh by a chromatic semitone. For instance, the interval from C to B is a major seventh, eleven semitones wide, and both the intervals from C to B, and from C to B are augmented sevenths, spanning twelve semitones. Being augmented, it is classified as a dissonant interval. However, it is enharmonically equivalent to the perfect octave. Since an octave can be described as a major seventh augmented by a diatonic semitone, the augmented seventh is the sum of an octave,
Regular tuning and temperament itself and its sharpened version, the chromatic semitone, is 7"f" − 4 of an octave. The common size of the intervals C–D, D–E, F–G, G–A and A–B, the whole tone, is the sum of the diatonic and chromatic semitones, or 2"f" − 1 of an octave. The diatonic semitone is larger than, smaller than, or equal to, the chromatic semitone according as "f" is less than, greater than, or equal to, 7/12. In the last case, the diatonic and chromatic semitones are both 1/12 of an octave, exactly half the size of a tone, and the tuning is 12-tone equal