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 mandolin Octave mandolin The octave mandolin is a fretted string instrument with four pairs of strings tuned in fifths, G, D, A, E (low to high), an octave below a mandolin. It is larger than the mandola, but smaller than the mandocello and its construction is similar to other instruments in the mandolin family. Usually the courses are all unison pairs but the lower two may sometimes be strung as octave pairs with the higher-pitched octave string on top so that it is hit before the thicker lower-pitched string. Alternate tunings of G, D, A, D and A, D, A, D