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 distinction is not crisp, and there are cases of each term being used in each country. Jimmy Moon, a Scottish luthier calls his version of the instrument by both names and Paul Shippey, an English luthier, uses the term octave mandolin. Confusion will likely continue as the terms continue to be used interchangeably. Octave mandolin construction is similar to the mandolin: the body may be constructed with a bowl-shaped back according to designs of the 18th-century Vinaccia school, or with a flat (arched) back according to the designs of Gibson Guitar Corporation popularized in the United States in the early