Why en passant exists: the pawn that cannot skip past you

En passant looks like a bug the first time you see it. A pawn advances two squares, lands beside yours, and on the next turn you capture it as if it had stopped on the square it passed. The capturing pawn moves diagonally onto that empty square; the opponent’s pawn disappears. Online boards still get angry tickets about “illegal” captures — Chess.com’s help centre even has a page titled that it is not an illegal move.

The problem the two-square advance created

In early European chess, pawns moved only one square. Between the 13th and 16th centuries, the optional two-square first move was added to speed the game. That change made it easier to create a passed pawn by jumping past an enemy pawn that would otherwise have captured on the adjacent file.

En passant (“in passing”) restores the capture opportunity the single-step world would have allowed. References appear in the 16th-century writings of Spanish master Ruy López de Segura. Wikipedia’s history notes it was one of the last major European rule additions; Italy long kept *passar battaglia* (no en passant) until adopting the international rule in 1880 ahead of the 1881 Milan tournament.

The modern conditions (plain language)

You may capture en passant only when all of these hold:

1. Your pawn is on the fifth rank (White) or fourth rank (Black) — the rank from which a diagonal capture would hit the square the enemy pawn skipped.
2. The opponent just moved a pawn two squares from its starting square, landing on the same rank beside yours.
3. You capture immediately on the next move. Skip the turn and the right is gone.
4. Only pawns capture or are captured this way.

FIDE’s Laws encode the capture in the pawn-move articles (commonly cited as Article 3.7 in recent editions). Notation may append e.p.; many digital boards simply show the destination square.

A practical tip for online play

On a browser board such as Board Atlas, watch the opponent’s unmoved pawns whenever your pawn sits on the fifth/fourth rank. Before you push a pawn two squares beside an enemy pawn, ask: “Do I want to gift them an immediate capture?” Before you decline an offered en passant, ask whether that pawn becomes a passer or opens a file against your king.

You do not have to take en passant — unless it is your only legal move (then it is forced, the same as any other only-legal move; 19th-century debates about “privilege” are settled). Taking is often right when it ruins a passer or opens a line; declining is fine when the capture wrecks your structure for no gain.

How this differs from castling

Castling (covered in our why castling exists post) solves king safety after the long-range queen/bishop reforms. En passant solves a pawn-structure loophole created by the double step. Both are special; only en passant captures on a square the victim does not occupy.