Légal Trap

There is 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 unsound. 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.

5... Bh5? (see diagram)

	a	b	c	d	e	f	g	h
8									8
7									7
6									6
5									5
4									4
3									3
2									2
1									1
	a	b	c	d	e	f	g	h

Position after 5...Bh5?

Black apparently maintains the pin, but this is a tactical blunder which loses at least a pawn (see below). Relatively best is 5...Bxf3 (or 5...Bd7), 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 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]

	a	b	c	d	e	f	g	h
8									8
7									7
6									6
5									5
4									4
3									3
2									2
1									1
	a	b	c	d	e	f	g	h

Final position after 8.Nd5#

The original game featured Légal playing at rook odds (without Ra1)[2] 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[3][4]

The above version is found in most publications with slight variation in move order and sometime a black pawn move h6 instead of g6. However, analysis by the Russian chess author Anatoly Matsukevich and the German chess player Stephan Maaß suggest that the game had been altered retrospectively in order to remove a flaw in the original game.[5] suggests that the move order has been altered Also the year 1750 is assumed to be wrong; it is more likely that the game was played in 1787, and that the original move order was:

1. e4 e5 2. Bc4 d6 3. Nf3 Nc6 4. Nc3 Bg4 5. Nxe5? Bxd1?? 6. Bxf7+ Ke7 7. Nd5# 1–0

Here the combination is flawed, as with 5... Nxe5 Black could have gained a piece. It is reported that Légal disguised his trap with a psychological trick: he first touched the knight on f3 and then retreated his hand as if realizing only now that the knight was pinned. Then, after his opponent reminded him of the touch-move rule, he played Nxe5, and the opponent grabbed the queen without thinking twice.

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."[8]

In general, setting up 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.

• Checkmate pattern

Bibliography

• Hooper, David; Whyld, Kenneth (1987). The Oxford Companion to Chess. Oxford University Press. ISBN 0-19-281986-0.

• Légal Trap at Chessgames.com