Légal's mate

Légal's mate usually arises from an opening trap. An enemy bishop appears to pin a knight to the queen. A pin occurs when moving one piece exposes another piece behind it. Because the exposed piece is the queen rather than the king, the knight may legally move when there is a strong concrete reason.

The attacker moves the knight and allows the opponent to capture the queen. If the opponent accepts without calculating, a bishop and both knights coordinate a sequence of checks that produces mate against the king still in the center. The idea is striking because the attacker gives up the most valuable piece and ends the game before the loss of the queen matters.

The queen is left behind as the net closes

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The knight on f3 blocks the diagonal between the bishop on g4 and the queen on d1: this is a relative pin.

An alignment that does not pin the king

The alignment compromises the queen, not the king. The knight on f3 may therefore leave the diagonal legally.

1. The knight on f3 blocks the diagonal between the bishop on g4 and the queen on d1: this is a relative pin.
2. The white king is on g1, outside that diagonal; moving the knight does not expose it to check.

Not every similar position permits the sacrifice. The full sequence must be checked, because the defender may decline the queen, capture something else, or create an escape square. Légal's trap is the sequence that offers the queen; Légal's mate is the final position with a bishop and two knights. Unlike Scholar's mate, it does not depend on an early queen invasion against f7.