En passant target square

When a pawn advances two squares from its starting position, it crosses an intermediate square. The en passant target square is that crossed square. If the opponent can capture en passant, the capturing pawn lands there while the pawn that made the double advance is removed.

In FEN, Forsyth-Edwards Notation, a text string describing a complete position, this information is the fourth field. It also appears among the four position fields before any EPD operation, where EPD means Extended Position Description. The value is a square name, formed by a file letter from a through h and a rank number from 1 through 8, such as e3, or a single hyphen when there is no target. It is always on rank three after White's double advance and rank six after Black's.

The original FEN specification records the square after every double pawn advance, even when no opposing pawn can capture. Some modern libraries use a narrower convention and include it only when an opposing pawn is positioned to make the capture, or when that capture is also fully legal. Two programs can therefore produce different strings for the same piece placement without disagreeing about the board.