By adding extra notes on top of the basic triads, it is possible to create various types of seventh chords. The 7th degree is either a major or minor third interval above the basic triad. A good knowledge of seventh chords is essential for any Jazz musician, as the chord progressions behind most Jazz songs are based on these chords. Here are the five basic types of seventh chords.
Notice that the diminished seventh chord contains a double flattened 7th degree (bb7). This note is actually the same as the 6th degree (A). It is 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.