Thanks pstring and trebledamage for your input. Let me recap in words that a beginning student might be able to grasp. Based on what trebledamage said in point #3, take the 7th note of the mixolydian mode and add it to arrive at the "regular" 7th chord. Would you agree? I'll save the relationship to the Gmaj scale for later. LOL's to pstring's humorous evaluation of the esthetic qualities of the mixo mode! It's one of those modes that we use all the time, and learn what it's called much later. That info does help, but it leaves me with a new question: why call that chord simply D7? Why not call it Dmixo7? Once again, Thanks!