I'm just starting to understand the modes and how they play their part in all music. I just picked up a chart for Eddie Harris' Cold Duck Time and saw the two chords D7 and G7. I was just wondering how these two chords fit together. What relationship do these chords have with each other? If the D7 is a mixolydian, shouldn't the 4 chord of that (ie G) be played with an Ionian which doesn't have a dominant 7? I know this is the case for hundreds of tunes but I'm just using this as an example.