Perhaps the most obscure and least used moves in Chess is called En Passant. It can only occur when a player exercises his option to move his pawn two squares on its initial movement. When this happens, the opposing player has the option to take the moved pawn "en passant" as if it had only moved one square. This option, though, only stays open for one move.

The en passant move was developed after pawns were allowed to move more than one square on their initial move. This was done to make sure they retained some of the restrictions imposed by slow movement, while at the same time speeding up the game.