Fifteen keys? I can only think of 12 major and 12 minor - A, Bb, B, C, Db, D, Eb, E, F,Gb, G, Ab. The scale of C has no sharps or flats, and the key of Gb (or F#) has six. Where are the other three keys, or are you saying that there's a difference between (for the sake of argument), Gb and F#? I've never found them on the guitar (or piano for that matter.) But I'm self taught............