The rule
From its starting square a pawn may move two squares at once. But if in doing so it “skips” a square attacked by an enemy pawn, that pawn may capture it as if it had moved only one square. This is en passant (French for “in passing”).
Conditions:
- the capturing pawn stands on the fifth rank (for White) or the fourth rank (for Black);
- an enemy pawn on an adjacent file has just moved two squares and landed next to it;
- the capture is allowed only immediately, on the very next move. If you miss the moment, the right is lost.
Example
1. 2. 3.
After 2… d5 the black pawn stands next to the white one. The white pawn captures it by moving to d6 — the square the black pawn “skipped”. The black pawn is removed from the board even though it was never on d6.
How it is written
En passant is written like an ordinary pawn capture: exd6. Sometimes “e.p.” is added.
Why this rule exists
Pawns used to move only one square at a time. When the two-square first move was allowed in the 15th century, en passant was added so that a pawn could not slip past an enemy pawn unpunished.
En passant is a frequent theme of chess puzzles and rules quizzes. So that you do not forget it in a game, after every two-square move by an enemy pawn, look whether one of your pawns stands next to it.