En Passant in Chess: When the Capture Is Allowed and Why It Exists

En passant, French for “in passing”, is a special pawn capture. When a pawn advances two squares from its starting square and lands directly beside an enemy pawn, that enemy pawn may take it as though it had moved only one square. It is the rule that newcomers meet last and most often doubt, because the captured pawn disappears from a square the capturing pawn never reaches.

The exact conditions

Two things must be true at the same moment. First, the enemy pawn has just advanced two squares in a single move. Second, the capturing pawn stands on the same rank, on an adjacent file, so that it attacks the square the enemy pawn passed over. In practice that means the capturing pawn is on its own fifth rank: rank 5 for White, rank 4 for Black. A White pawn on e5 can take a Black pawn that has just played d7–d5 or f7–f5. The capturing pawn then moves diagonally forward to the passed-over square, d6 or f6 in this case, and the enemy pawn is removed from d5 or f5.

The timing is strict. The capture is legal only on the move immediately following the two-square advance. If the player makes another move instead, the right is gone for good, even if the same pawns still stand beside each other. The capture is optional: a player may decline it unless it happens to be the only legal move.

Why the rule exists

Early chess had no two-square pawn step. It was added between the 13th and 16th centuries, and with it came a problem: a pawn could use the double step to slip past an enemy pawn that would otherwise have been able to capture it on the square in between. En passant restores the capture that the single step would have allowed, so the double step cannot be used to escape. The rule appears in the 1561 book of the Spanish master Ruy López de Segura. In parts of Europe, especially Italy, the rule was not used for a long time; the Italian practice was called passar battaglia, and Italy adopted the international rules, en passant included, in 1880, ahead of the 1881 Milan tournament.

A capture with an unusual landing square

Every other capture in chess ends with the capturing piece standing on the square of the captured piece. En passant is the only exception. The capturing pawn lands on the empty square behind the enemy pawn, and the enemy pawn is removed from the neighbouring square. Only pawns can capture or be captured this way. A knight, bishop or any other piece that passes a pawn’s double step has no such right, and a pawn cannot take a piece en passant.

How it is written

In algebraic notation the move is recorded by its destination square, so a capture from e5 to d6 appears as exd6, with the starting file in front as for any pawn capture. Adding “e.p.” is optional but common in books and annotations. A familiar line runs 1.e4 e5 2.Nf3 Nf6 3.d4 exd4 4.e5 Ne4 5.Qxd4 d5 6.exd6 e.p. The earliest possible occurrence is on move 3, as in 1.e4 e6 2.e5 d5 3.exd6 e.p., a sequence that was played in Steinitz against Fleissig at Vienna in 1882.

Legality and check

An en passant capture is subject to the ordinary rule that a player may never leave the own king in check. Because two pawns leave the same rank at once, a pin along that rank can make a capture illegal even though it looks harmless. In the position 8/6bb/8/8/R1pP2k1/4P3/P7/K7 with Black to move after d2–d4, Black’s pawn on c4 could take on d3, but both pawns would leave the fourth rank and the rook on a4 would then give check along it. The capture is therefore not allowed.

The capture can also give check in a way no other move can. In a 1928 game between Gundersen and Faul, 15.hxg6 e.p. delivered mate by double check. The Wikipedia article on the rule describes this as the only way a double check can arise without a checking piece moving. A player who has only the en passant capture as a legal move, and is not in check, is not stalemated; the capture must be played. Howard Staunton held this in his 1860 book Chess Praxis, and the rules were later amended to say so plainly. The largest number of en passant captures recorded in a single game is three.

The FEN field

A position written in FEN has a field for the en passant target square, the square the double-stepping pawn passed over. After 1.e4 it reads e3. The rank is redundant, since it can only be 3 or 6, and the file carries the real information. Some programs write the square after every double step and some only when a capture is actually possible, so two strings for the same position can differ. The FEN entry describes both conventions.

Repetition

The Laws of Chess define when two positions count as the same for threefold repetition. Article 9.2.3 requires that the same player has the move, and 9.2.3.1 adds that positions differ if at the start of the sequence a pawn could have been captured en passant. The right to capture is therefore part of the position. A position reached right after a double step, with a capture available, is not the same as the identical arrangement of pieces a move later, when the right has lapsed. Castling rights are treated the same way under Article 9.2.3.2: they matter, and they are lost only once the king or the rook has moved.

Engines and databases

For programmers en passant is a notorious source of bugs. Its target square differs from the square of the captured pawn, which makes move generation unlike that of any other capture, and the pin along the rank needs its own test. Hash keys must also reflect the en passant state, otherwise identical repeated positions can receive different keys when the first one followed a double step that allowed no capture. The Chess Programming Wiki records a case from the Aegon tournament of 1994, where a program pushed a pawn between two enemy pawns, created two possible captures and generated only one of them. The wider picture of how such programs work is on the computer chess page.

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