Opposition and Key Squares

Opposition is how you reach the key squares; key squares are why reaching them wins.

Opposition means the kings face each other on a file or rank with one square between them, and the side that does not have to move holds it. Holding the opposition means the opponent must step aside, which is the only way the attacker's king gains ground in front of the pawn.

Concrete placement one: kings on e6 and e8 with the pawn on e5. The kings are two squares apart on the e-file with one square between them, so whoever does not have to move holds the opposition. If it is the defender's move, the defender must give way and the attacker's king steps forward.

Concrete placement two: kings on e6 and e7 with the pawn on e5. There is no opposition here — no square between the kings — and the defender is holding the block square in front of the pawn. This is the drawn configuration the procedure must avoid.

Key squares for a pawn are the squares its king must reach to guarantee promotion; for a pawn on the fifth rank they are the three squares two ranks ahead of it. Occupy any one of them with the king and promotion follows whatever the defender plays.

The two ideas are one technique. Use the opposition to force the defender aside, step into the vacated ground, and the key squares fall into reach. Miss the opposition and the key squares stay on the far side of the defending king.

• Opposition: one square between the kings, defender to move.
• No opposition: kings adjacent, defender holds the block square.
• Fifth-rank pawn: three key squares two ranks ahead.
• Rook pawn: no key-square shortcut; the corner holds.

Further reading

• The Square Rule
• Convert the Won Endgame