Perhaps the most obscure and least used move in Chess is called en passant (pronounced “aw pawsawnt”). It can only occur when a player exercises his option to move his pawn two squares on its initial movement and that move places his pawn next to the opponent’s pawn. When this happens, the opposing player has the option to use his pawn to take the moved pawn “en passant” or “in passing” as if the pawn had only moved one square. This option, though, only stays open for one move.
In the example diagram, the light pawn is in position on black’s fourth rank. The dark pawn, makes an initial TWO space move. The light colored pawn can now capture the black pawn “in passing”. The light colored pawn actually moves behind the dark one. The dark pawn is removed. It would then be black’s turn. Had white failed to move on that turn, he would have lost the option to capture en passant.
The en passant move was developed in late medieval times after pawns were allowed to move more than one square on their initial move. This was done to make sure the now faster moving pawns retained some of the restrictions imposed by slow movement, while at the same time speeding up the game.