I know what a GFP outlet is! My point was that if you do any new house wiring today, that IS up-to-code, you have to use them, at least in places like the bathroom!
My other point was that there are a lot of AC cords on things that have the third prong and don't use it. This is a strong indication that it should only be plugged into an outlet with all three holes present and that it will only go in one way. Of course there is also the fact that sometimes one of the two spades is wider than the other. This has the same effect.
Most home stereo equipment is only two prongs.
My Fender Twin Reverb, all-tube guitar amp has three prongs and a switch to choose which way to ground the chassis. So do other guitar amps. Yes, I have gotten zapped from touching the bass player!
James.