VERY complicated chord structures on songs like I'll Be Home For The Holidays, Christmas Time Is Here, etc. After doing that kind of thing a couple of time, I know pretty much the entire fretboard. And even if I don't know off the top of my head that say, the 14th fret of the G string is A (which I do, but thats not the point), I know that 12th fret is the octave, and A is just two semitones above it.
