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,
Octave experience octave equivalency, and its biological basis apparently is an octave mapping of neurons in the auditory thalamus of the mammalian brain. Studies have also shown the perception of octave equivalence in rats , human infants , and musicians but not starlings , 4–9 year old children , or nonmusicians . Most musical scales are written so that they begin and end on notes that are an octave apart. For example, the C major scale is typically written (shown below), the initial and final C's being an octave apart. \override Score.TimeSignature #'stencil = ##f \relative c' { </score> Because of