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 also varies widely, from as few as 17 to as many as 24 frets: 18 or 19 frets is typical. From the mid-20th century on, a number of makers have produced octave mandolins with guitar-shaped (e.g., the "hourglass" or "figure-8") bodies. These instruments are typically constructed essentially like acoustic guitars, with similar woods and internal bracing. the neck, however, is much narrower, and supports the mandolin string layout, rather than the traditional 6-strings of the guitar. As with the mandolin and mandola, the octave mandolin has four courses of two strings each. The two strings in each course are tuned