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. E.g. 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 just called diminished chords and are usually played instead of a diminished triad wherever a diminished chord symbol occurs. Here are some common open diminished 7th chord shapes.