By adding extra notes on top of the basic triads, it is possible to create many other types of chords. Different types of seventh chords are created by adding another note either a major or minor third above the basic triad. The formulas for the five basic types of seventh chords are shown here.
The final type of seventh chord is the diminished seventh. This chord is unusual in that it contains a double flattened 7th degree ( bb7). This note is actually the same as the 6th degree (A) but it is technically called Bbb7 because the interval has to be some kind of seventh rather than a sixth because the chord is a type of seventh chord.