If a compostion is written in the Key of G (one F# at Treble Clef) and the chords are G7-C9-D9,why is it that you can play G mixolydian and G minor scales? and "Not" G Ionain and A dorian and on down the 7 modes in the Key of G? I think it would be bcause they are domaint chords and Mixolydian is the Domaint Mode.Im lost and I dont understand why you can play both Pentatonic and Major scales.Im used to a system of one mode for one chord. John, Palm Harbor, FL

Hi John
One mode per chord, eh? Hmmm, to me, you're cheating yourself out of a lot of potentially great sounds. To me, that's like cooking something only one way all the time. Think of scales as different flavors that can be applied to chords.

Here are some quick thoughts.

Over the G7, possibilities include G Mixo, G blues, G major or minor pentatonic, and more. The modes of G have an F# in them, which disagrees with the F natural in the G7 chord, except as a chromatic passing tone, or purposeful dissonance.

Over C9, one might choose C Lydian b7, G blues, minor pentatonic, C mixo, etc. Notice that all of these have a Bb in them. The movement from B natural to Bb and F to E are two of the strongest shifts in the progression G7 to C7. I would probably refer to them in my scale choices. Over the C9, you could use an F natural or F# or both. It's just a matter of flavor.

Hope this helps.

Edly
Edly is the author of "Edly's Music Theory for Practical People" and "Edly Paints the Ivories Blue." More info at http://www.edly.com/