Now Roger, I taught teenagers algebra, so I know you can learn a little music! As I told my sister when she was taking algebra, the first thing to do is to NOT THROW THE BOOK ACROSS THE ROOM when you first read an explanation.

Night Owl, in the key of C, the G chord IS the V (Five) chord. We use the V7 because it sounds good. Correct me here historians if this explanation is not quite right...historically people just started using what sounded good and then it got to be familiar to them they continued to use it. (Western music). I taught beginning piano students (even 4 year-olds) how to play the tonic chord then the V7 chord in the first 6 easy keys so they could play "Mary Had a Little Lamb" in 6 different keys. (In the key of C, the tonic chord is just the home chord, C...and the V7 chord is the G7 chord) I would also tell them to "drive Mom crazy" by playing a V7 chord and then walking away from the piano. It seems that our ear wants to hear it "resolve" or go to the home/tonic chord after it's played.

Hang in there. This stuff makes sense as we hear it explained in different words again and again.