Passing chords are used to link chords together in a progression. It is common to use diminished chords and minor 7 chords for this purpose.
Diminished chords are perhaps the most useful of all the substitutions, since they can be inserted between any two chords, and one diminished chord will cover four different keys (see Diminished Chords).
Furthermore, when we have a I-V progression, any diminished chord will fit in between. Here are some examples:
Minor 7 chords are also used in passing from one chord to another effectively. Here are some examples: