Years ago, my partner and I tuned a half-step down just to suit our voices. (We don't do it now, we've grown up.) So we'd play an "E" chord, but really be playing "F." and so forth. Well, we had a fiddler with perfect pitch play with us for a few months, who didn't know all our arrangements, so he watched our chording on the guitars while playing with us. It drove him nuts! He'd be seeing one chord, and hearing another, and trying to fiddle the right one. He didn't last.

Seamus