The diminished 7th chord is known as a symmetrical chord because it is made up entirely of the same interval (minor 3rds) stacked one on top of the other. Because of this, any note of the chord can be the root note. This means you never have to move up or down one fret to find the right chord. The main function of this chord type is as a passing chord (creating a link from one section of the harmony to another) and as a substitute for a dominant 7th chord. Here are some useful diminished 7th chord shapes and an example which uses them.
Fo
F#o
Bo
Go
Here are some examples making use of all five types of 7th chords. They can be played either with a pick or fingerstyle. Learn them, analyze them and transpose them to other keys.