The Rule of the Square

White to move. White king on c6, White pawn on e6, Black king on g1. The engine evaluates this as mate in 21 at depth 16 — once the pawn queens, the resulting K+Q vs K is a straightforward forced win. The correct first move is e7: push the pawn immediately. Black's best response is Kg2, then White plays Kc5, continuing to escort the pawn while the Black king scrambles far too late. The pawn will reach e8 and promote before Black's king can interfere.

You don't need to count individual moves. Draw an imaginary square:

1. Find the pawn's current square (e6 here).
2. Count the number of ranks to the promotion square (e6→e8 = 2 ranks).
3. Draw a square of that size toward the promotion rank. Here: e6→e8 (two up), then two files right → g8, then back down to g6. The square is e6–e8–g8–g6.
4. Can the enemy king step INTO that square on its next move? Black's king is on g1 — it cannot reach g6, g7, g8, f6, f7, f8, e6, e7, or e8 in one move. It is outside the square — the pawn promotes.

If the enemy king is already inside the square, or can step in on its move, it catches the pawn. If not, push with confidence.

The rule works because a king and a pawn both move exactly one square per turn. The pawn needs N moves to promote; the king needs to reach any square in that N×N box within N moves to intercept. Counting the box is a pattern, not a calculation — once you internalize it, you spot it in two seconds. The key practical detail: it's Black's move that matters for stepping in. If the side with the pawn moves first (as here), count the square for the current position and ask whether the enemy king can step in on its upcoming turn. If it's the defending side's turn to move, the king may catch a pawn it otherwise couldn't — always check whose move it is.

Three situations where the rule of the square doesn't give the full answer:

• A rook's pawn (a- or h-file). A king that catches an a- or h-file pawn can still draw if it reaches the promotion corner, because the defending king traps the winning king in stalemate patterns. The square rule tells you the pawn promotes — it doesn't guarantee a win.
• The defending king is inside but blockaded. Pieces or pawns can obstruct the king's path even inside the square. Always verify there's a clear route.
• Whose move it is. The single most common mistake: forgetting that a one-move difference flips the outcome entirely. Always identify the side to move before applying the rule.

In this specific position none of these exceptions apply — the pawn is on the e-file, Black is to move next, and there are no obstructions. Push e7 and escort it home.