En passant

En passant is a special move in chess whereby a pawn is able to capture an opposing pawn that tries to jump past the ordinary diagonal for pawn captures. This move is rarely available, but can be extremely effective when used. It requires the following conditions:

• The captured pawn must have jumped two squares in its opening move, to align itself with the capturing pawn.
• The capturing pawn must have advanced only three ranks to its position prior to the capture.
• The capture must occur at the first available opportunity, not later.

This is not allowed if the pawns are not next to each other, or if the capturing pawn has moved beyond three squares from its originating position, or if the opposing pawn has moved twice.