and the king cannot move into the square, so the pawn queens. (See Wikibooks – Chess/The Endgame for further discussion on the rule of the square.) Even if the defending king can move inside the square of the pawn, the attacking king may be able to block it, as in the diagram from Fishbein. 1... Ke4 Moving into the square. 2. Kb4 Kd5 3. Kb5! Kd6 4. Kb6! Kd7 5. Kb7! Kd6 6. a5 Kc5 7. a6 Kb5 8. a7 and the pawn promotes. Note that in some cases, the king can catch a pawn when he is outside the square by creating threats that must be parried, and gain a tempo. In the Réti endgame study (by Richard Réti, 1921), the white king is outside the square of the black pawn, two tempi short of catching the pawn. However, White can draw by "going after two birds at once". 1. Kg7! h4 2. Kf6! Kb6 If 2...h3 then 3.Ke7 or 3.Ke6 and the pawns promote together. 3. Ke5! Kxc6 If 3...h3 then 4.Kd6 h2 5.c7, draw. 4. Kf4, resulting in a draw. Key squares If the defending king is within the "square", then the pawn cannot queen without the help of its own king. The first concept that needs to be introduced is that of the key square, also known as a critical square. A key square is a square such that if White's king occupies it, White can force the pawn to promotion, regardless of where the black king is and regardless of which side is to move, and against any defense (assuming that the black king cannot capture the pawn). The key squares are relative to the position of the pawn. Whether or not the white king can reach a key square depends on the position of the pieces. Of course, even if the white king occupies a key square, accurate play is still required in order to promote the pawn. Note that the key square is in front of the pawn. Endgame expert Yuri Averbakh said, just as a father leads his child across the road rather than pushing the child in front, the king should also lead the pawn to the queening square. Rook pawn A rook pawn (on the a- or h-file) has much less chance of promoting than other pawns. The reason is that if the opposing king can get to any square