Originally posted by Kunsoo
[b]When was this rule introduced into the game? More importantly: why?
When chess was first invented (believed to be late sixth century A.D.)the pawn could move one square straight forward if that square was unoccupied, or could capture one square diagonally forward (these options are of course still available).
To speed the game up in the opening, the double square first move for pawns was introduced, (I think in the 12th or 13th century), but it was felt that this should not allow a pawn using the double move to
avoid a capture by an opposing pawn which was on it's own fifth rank. So the 'En Passant' capture was introduced some time later, (14th century?) to allow a capture as if the pawn had only moved one square, and since under the original laws it could have moved to it's fourth rank after a second move, this is why the capture is allowed only for one move. I hope this helps. I found it easy to remember this rather unusual law once I knew the reason for it.