As with half-step approach chords, passing chords are subsidiary to the main progression. Unlike half-step approach chords, however, passing chords pass by step between two chords. Consider the following:
Here, the F sharp dominant seventh chord is a half-step approach chord while the F sharp diminished seventh chord is a passing chord, as it passes between the dominant (G) and subdominant (F) chords.
An entire harmonised scale may be decorated with passing chords
Any progression, therefore, which includes diatonic chords in succession,
can be decorated with passing chords
Passing chords can also substitute for other chords in a progression. For example, in a I – ii – V – I progression, with each chord sounding for a whole bar, a passing chord may be used for part of the initial tonic bar
In a I – VI – ii – V progression, a passing chord often substitutes for the VI chord.
Here, the F sharp diminished seventh can be heard as both a passing chord – passing between F and G minor – and as a substitution for the submediant (VI).