Hmm. A little hard to explain, and I will probably get it wrong. It is like the rules about being able to substitute one chord for another -- like the I-VI-iii; IV-VI-ii; V-iii-vii chords, because there are notes from one in the other. You can hear Dylan doing this in some of his accompaniments when he hasn't got the chord he needs in the harmonica. Can't think of an example at the moment.

yours,

Peter T.