Pawns move straight forward one square, but capture diagonally forward one square. On its first move, a pawn may advance one or two squares. Pawns never move backward and cannot capture straight ahead, so a piece directly in front blocks them completely. Two special rules involve pawns: en passant, a capture of a pawn that has just moved two squares, and promotion, where a pawn reaching the far end becomes another piece. Though the weakest units, pawns form the skeleton of every position, so think carefully before making pawn moves you cannot take back.