Originally posted by Fat Lady I'm sure I saw a chess problem once where the key move was e8=R and then mate was forced a few moves later by o-o, o-o-o or o-o-o-o! (the last one moving the king two squares towards the rook on e8, i.e. to e3, and the rook moving the other side of the king, i.e. to e2).
^ Without this additional restriction, which was added to the FIDE rules in 1972, it would be possible to promote a pawn on the e file to a rook and then castle vertically across the board (as long as the king has never moved and there are no pieces in between). This way of castling was "discovered" by Max Pam and used by Tim Krabbé in a chess puzzle before the rules were amended to disallow it. [1]
Here is the link...sorry...couldn't find it in English