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. An interval is a measure of distance between two notes. For more on intervals, see Progressive Music Theory.
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.