Seventh chord an augmented fifth, and an augmented seventh (i.e., a sequence of 3 major thirds, such as C–E–G–B), is a rarely used tertian seventh chord. The reason is that the augmented seventh interval is enharmonically equivalent to one entire octave (in equal temperament, 3 major thirds = 12 semitones = 1 octave) and is hence perfectly consonant with the chord root. The seven commonly used chords are: Seventh chords can also be constructed using augmented or diminished thirds. These chords are not tertian and can be used in non-tertian harmony. There are many (mathematically, 64) chords that can be built, however,
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