Légal’s Mate is checkmate pattern named after the French chess player, Sire de Légal (1702–1792). He commonly used the pattern in his games. Since it is actually an opening trap, it’s also known as Légal’s Trap.
Légal’s Mate Example
Diagram above: White plays the surprising move 1.Nxe5! Doesn’t this move blunder the queen since black can now play 1… Bxd1?
If black plays 1… Bxd1? then white will execute a forced checkmate (which I’ll show you in a moment).
Instead, black’s best try is to play 1… Nxe5 (threatening 2… Nxc4) and we will reach the position in the diagram below:
Diagram above: Instead of 1… Bxd1, black might play 1… Nxe5. White should then play 2.Qxh5 (regaining the lost piece) and if black plays 2… Nxc4 then 3.Qb5+, followed by 4.Qxc4 leaves white a pawn ahead.
Let’s return to the main question–what happens after the moves 1.Nxe5 Bxd1? We then reach the position in the diagram below:
Diagram above: 2.Bxf7 initiates the Legal’s Mate combination! Black is forced to play 2… Ke7, after which 3.Nd5# is checkmate.
Diagram above: 3.Nd5# White just gave up their queen, but Légal’s Mate ends the game!
Interesting Notes on Legal’s Mate
According to Wikipedia, Légal disguised this trap with a psychological trick:
Diagram above: In this position, Légal apparently touched the knight on f3 and only then, after he touched it, pretended to notice that the knight was pinned! In this way he tricked his opponent–who then insisted on the touch-move rule–after which Légal “reluctantly” played Nxe5. Many amateur player’s would definitely be tempted to capture the white queen, but it loses on the spot.
Légal’s Mate is sometimes referred to as Blackburne’s Trap, since J Blackburne (1841–1924), a British chessmaster, also set this trap on many occasions.
The idea to unexpectedly move a pinned piece is quite common in chess. An important lesson here is that you shouldn’t assume a piece won’t move just because it’s pinned (unless it’s pinned to the king of course). Légal’s Mate is a good example of this.