En-passant

You learnt earlier that, although you can normally only move a pawn one square forward at a time, you can choose, if you wish, to move a pawn two squares forward on its very first move.

Originally, this rule did not exist and you could never move a pawn more than one square forward at a time, However, it was felt that the start of each game was a bit slow and boring, as it was taking a lot of moves before the pawns could start capturing each other. It was generally agreed that it would save time and liven things up a bit if you were allowed to move a pawn either one or two squares forward on its very first move. However, there was one slight problem with this idea. You see, when you could only move a pawn one square forward at a time, your opponent could simply wait with another pawn on the next file and that single pawn was enough to prevent you from ever moving your pawn all the way forward unopposed, as your pawn has to occupy the square in front of the enemy pawn at some stage along the way, so it can be captured at least once along the way. But now, being able to move your pawn either one or two squares on its very first move, if your opponent has a pawn on the next file waiting to capture your pawn if you move it one square, and you move it two squares instead, the en=passant rule states that your opponents pawn may still capture your pawn as if you had moved it just one square forward instead.