The diminished 7th chord is unusual in that it contains a double flat 7th degree (~7). There are three possible symbols for this chord: o, o7 or dim7.
The following example demonstrates a C diminished 7th arpeggio (Co). Since the note B~ is enharmonically the same as A natural, it is written here as A to make it easier to read.
Diminished chords are commonly used as passing chords which create tension and then resolve to the next chord, or as substitutes for dominant 7ths chords. In the following example, the Bo is a passing chord between chords 1 and 2, while the Ao is a substitute for 5.