as a guitarist CAGED covers the keys we play in and a capo sorts the rest out. however a friend of mine plays piano and apparently the way notes fall under the fingers is very different. if you listen to Johnny B Goode by Chuck Berry it's in Bb on the record. But we all play it in A. I wonder if Chuck tuned down half a step for the benefit of the keyboard player as Bb was described by Keith Richard in an interview as a piano player's key.