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.
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
4. Nc3 Bg4?!
5... Bh5? (see diagram)
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.
The original game featured Légal playing at rook odds (without Ra1) against Saint Brie in Paris 1750:
Black springs Légal's Trap on White
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||
The "Sea-Cadet" Mate
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.