My son's new piano teacher is all about jazz. He took the melody line from the song Toyland, and told my son to play that with his R hand and various chords (single bottom note then chord-chord in 3/4 time) with the L hand. He wrote in which chords to use. But in one spot he wrote G7 = DFGB and in another G7= DFGBb.

The song (is it a piece?) is in the key of F major. Is a G7 chord always one way or the other, or can it contain either B or Bb depending on what else is in the measure?

Hope I'm explaining this right, I don't have a scanner to put in the sheet music. Thank you!

All I know is that if there is a Bb in the G chord, as you said (DFGBb), that would make it a Gminor7 chord. So, it's still a G7, just a minor G7 - the third (B) is flatted.

"Song" is kool.
G7 = D F G B is correct.
Gmin7 = D F G Bb. The flattened 3rd (the Bb) gives the minor characteristic to this chord.

A-ha! There is a little "-" sign, he must have meant G minor and I didn't see it. When I asked my son if the teacher said anything about the minus sign, he said "oh yeah..."

Thank you!

The minus looking sign is the sign for minor.

So G7 is a major sounding chord and G-7 is the minor one (I can't find the exact right sign on my laptop - the minus should be up higher, but you get the idea).