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 standard 88-key piano keyboard, while the C an octave higher is C. The notation "8" or "8" is sometimes seen in sheet music, meaning "play this an octave higher than written" ("all' ottava": "at the octave" or "all' 8"). "8" or "8" stands for "ottava", the Italian word for octave (or "eighth"); the octave above may be specified as "ottava alta" or "ottava sopra"). Sometimes "8" is used to tell the musician to play a passage an octave "lower" (when placed under rather than over the staff), though the similar notation "8" ("ottava bassa" or "ottava sotto") is also used.