Lesson
White to move. Main line: 1. e4 e5 2. Bc4 d6 3. Nf3 Bg4 4. Nc3 g6 5. Nxe5! Bxd1 6. Bxf7+ Ke7 7. Nd5#
The trap turns on the pin that isn’t. Black’s bishop on g4 “pins” the f3-knight against the queen — but a pin against anything except the king is only a suggestion. Nxe5 breaks it, and the greedy 5...Bxd1 loses to a forced mate: the bishop check drags the king to e7, and Nd5 covers every flight square with help from the e5-knight and Black’s own d6-pawn.
The honest defense 5...dxe5 avoids mate but concedes a clean pawn after Qxg4 — the bishop on g4 was only ever defended by the illusion of the pin.
This line is the classic argument for checking every capture: material counting said Bxd1 wins a queen; the mating net said otherwise. As a replay with both branches recorded, viewers see the trap and the refusal side by side.
If 5...dxe5 (declining): 5... dxe5 6. Qxg4 — White has simply won a pawn.
The oldest trap still worth knowing
The trap is named after Sire de Légal, an eighteenth-century Parisian player, which makes it one of the oldest named opening traps in the game. It still catches players today, which is remarkable for an idea this old.
The mechanism is a queen offered as bait and a mate delivered by three minor pieces — bishop, knight and knight — against a king that never castled.
The pin that was never real
Black pins the f3 knight against the queen with ...Bg4, and White ignores it: Nxe5. If Black takes the queen with ...Bxd1, White mates with Bxf7+ and Nd5#.
The point is that a pin against a queen is not a pin at all. A pinned piece may not move only when the piece behind it is the king. Everything else is a cost-benefit question, and here the queen is worth less than the mate.
What the engine says about the whole line
Checked move by move, White’s play in the recorded line is sound but not forced: the early moves give up a few hundredths, and 5.Nxe5 evaluates at about +2 — a good move on its own terms even if Black declines the bait.
That is worth stating plainly. Taking the queen is the losing move; declining it leaves Black merely worse rather than mated. The trap does not win by force, it wins by temptation.
5.Nxe5: About +2.0 and the engine’s top choice — sound whether or not Black takes.
5...Bxd1 (taking the queen): Loses immediately to 6.Bxf7+ Ke7 7.Nd5#, which the engine confirms as forced mate.
Declining the queen: Black stays in the game a pawn down. The trap is a practical weapon, not a refutation.
Only a king behind a piece creates an absolute pin. Against a queen it is a suggestion.
Before capturing material offered in the opening, count the attackers around your king.
Three minor pieces can mate an uncastled king without any heavy pieces at all.
Source: Légal Trap — https://en.wikipedia.org/wiki/L%C3%A9gal_Trap