The diminished chord is made up of four notes. When compared to the major scale they are:
So once again if we look at the A major scale:
We find that 'A diminished' would be composed of the notes A (1st), C (b3rd), Eb (b5th), and Gb (bb7th). The fingering would be:
The diminished chord has a unique property in that the root, b3rd, b5th and bb7th will share the same notes across those four keys, for example:
So one diminished chord can cover four keys, since it will contain exactly the same notes in each of those four keys. This is very useful when changing keys during a piece of music, and they are also used as transition chords to pass from one chord to another in a progression, and as a substitution for dominant 7th chords.