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,
Eleventh perfect fourth. Eleventh In music or music theory an eleventh is the note eleven scale degrees from the root of a chord and also the interval between the root and the eleventh. The interval can be also described as a compound fourth, spanning an octave plus a fourth. Since there are only seven degrees in a diatonic scale the eleventh degree is the same as the subdominant. The eleventh is considered highly dissonant with the third. A perfect eleventh is an eleventh which spans exactly 17 semitones. It can be also described as a compound perfect fourth, spanning an octave