What is "wrong rook pawn"

n. (context chess English) In the endgame, a rook pawn that is unable to promote under the protection of a bishop, because the promoting square is of the color the single friendly bishop cannot control.

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In chess endgames with a bishop, a pawn that is a rook pawn may be the wrong rook pawn. With a single bishop, the result of a position may depend on whether or not the bishop controls the square on the chessboard on which the pawn would promote. Since a side's rook pawns promote on opposite-colored squares, one of them may be the "wrong rook pawn" . This situation is also known as having the wrong-colored bishop or wrong bishop, i.e. the bishop is on the wrong colored squares in relation to the rook pawn . In many cases, the wrong rook pawn will only draw, when any other pawn would win. A fairly common defensive tactic is to get into one of these drawn endgames, often through a sacrifice.

In some endgames such as having a bishop and pawn versus a lone king (perhaps with pawns), the wrong rook pawn is the one whose promotion square is the opposite color as that on which the bishop resides, which makes the stronger side unable to win. This was known at least as early as 1623 because of an endgame study by Gioachino Greco (see below).

A less-common situation is when the defense has a bishop versus a rook and rook pawn; the wrong rook pawn is the one that promotes on the square not controlled by the bishop because the defending king and bishop can form a blockade in the corner (on the pawn's promotion square) and draw the game. (This is also called the safe corner for the defending king.)