The Saavedra position: promote to a rook

It is a position with very few pieces: a white king and pawn against a black king and rook.

White's pawn is on c6 and it wants to reach c8. Black's rook is trying to stop it, and Black's king is far away in the corner on a1.

The position is famous for one move at the end. The pawn reaches the last rank and White chooses a rook rather than a queen.

White starts with 1.c7. The rook stops the pawn with 1...Rd6+, a check that also covers c6.

Now the white king walks down the board and the rook checks it every step.

1. 2.Kb5 Rd5+
2. 3.Kb4 Rd4+
3. 4.Kb3 Rd3+
4. 5.Kc2

On c2 the king stands right beside the c-file, so the rook has run out of useful checks. Only the second move needs care. If White plays 2.Kc5 the rook goes to d1 and stops the pawn. If White plays 2.Kb7 the rook goes to d7 and takes it.

Black plays 5...Rd4. It looks like giving up. It is the cleverest move in the position.

The rook is offering itself. If White makes a queen, Black plays the rook to c4 with check, the queen must take it, and Black has no legal move left.

A player who has no legal move and is not in check is stalemated, and a stalemate is a draw. All that work would be thrown away by the strongest piece on the board.

Play it out and look at the board. White has made a queen, Black has checked on c4, and the queen has captured.

The black king on a1 is not in check. It cannot go to a2, because the queen on c4 covers a2. It cannot go to b1 or b2, because the white king on c2 covers those two squares. Black has nothing else to move.

That is stalemate. The game is a draw, and White has a queen for nothing.

6.c8=R is the move. Black can offer the rook again with 6...Rc4+, but this time White takes with 7.Rxc4 and the black king simply steps to a2. There is no stalemate.

The new rook is also ready to swing across to a8 and give checkmate. Black's only answer is 6...Ra4, which guards the a-file.

Then 7.Kb3 makes two threats at once. The king can capture the rook on a4, and the rook can play Rc1 with checkmate. Black cannot stop both, so White wins.

When a pawn reaches the last rank it is exchanged for another piece. The player chooses a queen, a rook, a bishop or a knight of the same colour.

Choosing anything other than a queen is called underpromotion. In real games it is rare, and there are only two normal reasons for it.

• A knight, because a knight can give a check or a fork that a queen cannot.
• A rook or a bishop, because a queen would leave the other side with no legal move and the game would be drawn by stalemate.

The Saavedra position is the second reason in its clearest form.