I've played a lot of chess variants of various levels of sophistication. Here's one due to a friend of mine that I've been enjoying recently:
- normal chess, except you can't move the same piece your opponent just moved. So if I move a pawn, you must move a king/queen/bishop/rook/knight.
- check and checkmate are as normal, no need to capture the king. It's checkmate if the only move that would stop check is to move the forbidden piece. Stalemate also happens - king and queen is not enough to mate unless the kings are fortunately placed.
- castling is a king move
- pawn promotion counts as a pawn move when promoting, but a move with the promoted piece when considering the next player's move
- Black's e-pawn starts on e6 (alternatively, both players start the game with one free move)
My friend invented this as a way to play against beginners, where he plays with this restriction unknown to them. We refined it into an interesting competitive variant.
In regular chess it's incidental. It only comes up in relation to the touch-move rule and similar rules. And even there it's given special treatment: if you touch your king, then your rook, you must castle on that side if it's legal, instead of making any other legal king move.
In this variant, whether castling is considered a king move or a rook move or both has an effect on which moves are legal, so it needs to be spelt out.
- normal chess, except you can't move the same piece your opponent just moved. So if I move a pawn, you must move a king/queen/bishop/rook/knight.
- check and checkmate are as normal, no need to capture the king. It's checkmate if the only move that would stop check is to move the forbidden piece. Stalemate also happens - king and queen is not enough to mate unless the kings are fortunately placed.
- castling is a king move
- pawn promotion counts as a pawn move when promoting, but a move with the promoted piece when considering the next player's move
- Black's e-pawn starts on e6 (alternatively, both players start the game with one free move)
My friend invented this as a way to play against beginners, where he plays with this restriction unknown to them. We refined it into an interesting competitive variant.