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How did en passant become part of the rules of chess?

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A chessboard showing the standard starting position for a game, with white and black pieces arranged in two rows on each side: rooks, knights, bishops, queen, and king on the back row, and pawns in front. — chess Position of chess pieces at the beginning of a game. The pieces are placed on the two ranks closest to the player. In the closest rank, pieces are arranged from left to right: rook, knight, bishop, queen, king, bishop, knight, and rook. In front of that rank, each square has a pawn.

The rule of en passant was introduced to maintain the strategic balance of the game after the pawn was granted the option to move two squares on its initial turn.12

When pawns were restricted to moving only one square at a time, they could not bypass an enemy pawn's reach. By allowing a two-square opening move, the rules created a scenario where a pawn could "skip" over a square controlled by an opponent. En passant—French for "in passing"—was implemented to ensure that a pawn could still be captured as if it had only moved one square, preventing players from using the two-square jump to evade capture.2

Although this rule was known as early as the 15th century, it did not gain widespread acceptance until the 18th century. It remains a unique, situational rule that must be exercised immediately after the opponent's two-square pawn advance, or the right to do so is lost.12