A diminished 7th chord is created by flattening the 7th degree of any minor 7 flat five chord. Because the 7th is already flattened, it becomes a double flattened 7th. For example, a C dim7 chord contains the notes C, Eb, Gb and Bbb (1, b3, b5 and bb7). A double flattened seventh is the same as a 6th degree (i.e., Bbb = A). It is referred to as a double flat 7 because the chord is technically a type of 7th chord. Diminished 7ths are commonly called diminished chords and are usually played instead of a diminished triad wherever a diminished chord symbol occurs.
Cº7
Dbº7
Dº7
Ebº7
Eº7
Fº7
F#º7
Gº7
Abº7
Aº7
Bbº7
Bº7