Modal interchange is the concept of replacing or borrowing diatonic chords from a 'parallel' scale and utilizing them in whatever key your composition is in at the moment. Be sure to bone up on 'keys of the moment.' Modal interchanges can be found in both major and minor pieces. For example, let's say you're playing in the key of F minor and you run into a G-7 chord. What's up with that. In the key of F minor, the ii or 'two chord' is a G-7(b5) not a plain G-7. The G minor from the natural minor key has been dumped in favor of the G minor from the melodic minor key. Let's say you were playing in the key of F major and you ran into a Bb minor or some Bb chord of whatever flavor...maybe a minor 7. Why would there be a Bb minor chord when the Bb 'should' be a major 7 chord (in the key of F maj)? Because the 'normal' Bbmaj has been thrown out for the moment and replaced by a Bb from the natural minor key. Other weirdness: The natural minor chords i minor, i-7, bIII, and bIIImaj7, can be used as interchange chords in major keys. Additionally, the bVIImaj7 chord can be used even though it is not diatonic to any minor key. The bIIma7 can be used as well. Use this spiffy diagram I drew to work it all out:
