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Légal Trap

The Légal Trap or Blackburne Trap (also known as Légal Pseudo-Sacrifice and Légal 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 (1702-92). Joseph Henry Blackburne (1841-1924), a British master and one of the world's top five players in the latter part of the 19th century, set the trap on many occasions.

Natural move sequence

There are a number of ways the trap can arise, the one below shows a natural move sequence from a simultaneous exhibition in Paris. André Cheron, one of France's leading players, won with the trap as White against Jeanlose:

1. e4 e5 2. Nf3 Nc6 3. Bc4 d6

The Semi-Italian Opening.

4. Nc3 Bg4?!

Black pins the knight in the fight over the center. Strategically this is a sound idea, but there is a tactical flaw with the move.

5. h3

In this position 5.Nxe5? would be an unsound trap. While the white queen still cannot be taken (5...Bxd1??) without succumbing to a checkmate in two moves, 5...Nxe5 would win a knight (for the pawn). Instead, with 5.h3, White "puts the question" to the bishop which must either retreat on the c8-h3 diagonal, capture the knight, be captured, or as in this game, move to an insecure square.
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After 5...Bh5?
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Légal's Mate: 8.Nd5#

5... Bh5? (see diagram)

Black apparently maintains the pin, but this is a tactical blunder which loses at least a pawn (see below). Relatively best is 5...Bxf3, surrendering the bishop pair and giving White a comfortable lead in development, but maintaining material equality. 5...Be6!? is also possible.

6. Nxe5!

The tactical refutation. White seemingly ignores the pin and surrenders the queen. Black's best course now is to play 6...Nxe5, where with 7.Qxh5 Nxc4 8.Qb5+ followed by 9.Qxc4, White remains a pawn ahead, but Black can at least play on. Instead, if Black takes the queen, White has checkmate in two moves:

6... Bxd1?? 7. Bxf7+ Ke7 8. Nd5#

The final position (see diagram) 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.

Légal versus Saint Brie

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

Other variations

Black springs Légal's Trap on White
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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# 0-1
 The Trap in a modern middlegame
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Position after 13...Nc6-e7? after a Ruy Lopez Bird's Defence opening from the game Short vs. Kupreichik, Hastings 1981-82. White gained a pawn by 14.Nxe5 and subsequently won.
 The "Sea-Cadet" Mate
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A version of Légal's Mate occurred after 7...Ne5 in a casual game won by Falkbeer at Vienna in 1847. The game was arranged as a display of living chess in Act II of Der Seekadet, an 1876 operetta by Genée and Zell:
1.e4 e5 2.Nf3 Nc6 3.d4 exd4 4.c3 (the Göring Gambit) dxc3 5.Nxc3 d6 6.Bc4 Bg4 7.0-0 Ne5 8.Nxe5 Bxd1 9.Bxf7+ Ke7 10.Nd5# 1-0

Considerations

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."

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.

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