The Légal Trap or Blackburne Trap (also known as Legal Pseudo-Sacrifice and Legal Mate) is a chess opening trap, characterized by a queen sacrifice followed by checkmate with minor pieces if Black accepts the sacrifice. The trap is named after the French player Sire de Légal (1702–1792). Joseph Henry Blackburne (1841–1924), a British master and one of the world's strongest players in the latter part of the 19th century, employed the trap on many occasions.
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There are a number of ways the trap can arise, one of them being:
1. e4 e5
2. Nf3 Nc6
3. Bc4 d6
4. Nc3 Bg4?!
5. h3
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5... Bh5?
6. Nxe5!
6... Bxd1??
7. Bxf7+ Ke7
8. Nd5#
The final position is a pure mate, meaning that for each of the eight squares around the black king, there is exactly one reason the king cannot move there.[1]
The original game featured Légal playing at rook odds (without Ra1) against Saint Brie in Paris 1750:
1. e4 e5 2. Nf3 d6 3. Bc4 Bg4?! 4. Nc3 g6 5. Nxe5 Bxd1? 6. Bxf7+ Ke7 7. Nd5# 1–0[2][3]
At a simultaneous exhibition in Paris, André Cheron, one of France's leading players, played a similar trap in the game Cheron vs. Jeanlose:
1. e4 e5 2. Nf3 d6 3. Bc4 Nc6 4. Nc3 Bg4?! 5. h3! Bh5? 6. Nxe5!
6... Bxd1?? 7. Bxf7+ Ke7 8. Nd5# 1–0
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Sometimes the mate can be administered by a different piece. This game from the Petrov's Defence is very old:
1. e4 e5 2. Nf3 Nf6 3. Nxe5 Nc6?! 4. Nxc6 dxc6 5. d3 Bc5 6. Bg5? Nxe4 7. Bxd8?? Bxf2+ 8. Ke2 Bg4# 1–0 (see diagram).
This kind of mate, where an apparently pinned knight moves anyway, allowing capture of the queen, but leading to a checkmate with minor pieces, occasionally occurs at lower levels of play, though masters would not normally fall for it. According to Bjerke (Spillet i mitt liv), the Légal Trap has ensnared countless unwary players. One author writes that "Blackburne sprang it several hundreds of times during his annual tours."[4]
In general, making a "trap" by luring a bishop into a queen capture is not strictly necessary. Any game featuring an advanced knight and Bxf7+ (or ...Bxf2+) followed by mate with minor pieces would be considered a Légal Mate. The mate succeeds because the square of the advanced knight is unguarded, and the enemy king is blocked by several of its own pieces.