Playing a mode in a solo is just using the notes from a mode. Say your going to Solo over a G major chord, and you start your solo with the note D over that chord.
Now say your solo part goes D to E to F# to G (over the G major chord).
If you did that you would be playing in the G Ionian mode (or Major scale).
Well what happens when you change a note in the sequence.
Say you go D to E to F to G.
Now your playing in the G Mixolydian mode.
See what I'm saying.
The Key signature of a Song depends on the melody and not the harmony, which is why there is alot of confusion in modes.
The same applies to chords, the melody can be one key, but you can borrow chords modally in the same way.
An example would be a lead in C major, but using an A minor chord to harmonize it. That's a simple answer, and sounds like just alittle chord substitution. Modal harmonies work the same way.
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