The rule of the square: can the king catch the pawn?

A file is a column of squares running up the board. A rank is a row running across it.

When a pawn reaches the far end of its file it is promoted. You take the pawn off the board and put a new piece on that square, almost always a queen. That last square is the pawn's promotion square.

The rule of the square answers one question. Can the enemy king get to the pawn in time to stop it?

Here is the rule. Draw a square on the board with one side running from the pawn to its promotion square. If the defending king can step inside that square on its turn, it catches the pawn. If it cannot, the pawn promotes.

The best part is that you never have to count moves. You just look.

Do it with your eyes, in three steps.

1. Start on the pawn's square.
2. Step diagonally towards the enemy king, one square at a time, until you reach the last rank.
3. The file you land on is the far edge of the square.

In the diagram above the pawn is on a4. Stepping diagonally gives b5, c6, d7 and then e8. So the square reaches the e-file, and its four corners are a4, a8, e8 and e4.

Now look at the enemy king. Is it inside those corners, or can it step inside on its move? If yes, the pawn is caught. If no, the pawn queens.

Every time the pawn moves forward, the square gets one file narrower. That is why the side with the pawn wants to push it straight away.

After 1.a5 the corners are a5, a8, d8 and d5. Of the squares the black king can reach, the nearest to the pawn are e5, e4 and e3, and none is inside the new square.

So 1.a5 wins the race. Any other first move by White lets the black king into the square, and then the pawn is caught.

Now give Black the move in the same position. The pawn is still on a4, so the square still reaches the e-file.

The king plays 1...Ke5 and walks diagonally towards the pawn's path. From inside the square the king can follow the pawn all the way down the board and take it.

This is the point every learner should hold on to. The position has not changed at all. Only the turn has changed, and it turns a win into a draw.

The rule is exact, but it assumes only three things on the board: two kings and one pawn. Watch for these three cases.

• The pawn is still on its starting rank. A pawn on a2 may move two squares at once. Draw the square from a3 instead, because the pawn is really one move ahead of where it stands.
• Something is in the king's way. If a piece or a pawn blocks the shortest path, the king needs extra moves and the rule gives the wrong answer.
• The other king can help. Even when the defending king reaches the pawn, the attacking king may be close enough to guard it. Then the pawn is not lost.

The common mistake is counting the pawn's moves and the king's moves one at a time. In a hurry the count comes out wrong by one, and one move is the whole game.

The second mistake is forgetting the double step from the starting rank. A pawn on h2 catches out a lot of people, because it is really only six moves from h8, not seven.

The third mistake is forgetting whose turn it is. Before you say the word draw, ask who moves next.