Legal’s mate is a tactical combination in which a player sacrifices their queen to deliver checkmate using minor pieces — typically two knights and a bishop — by breaking a relative pin on the knight on f3.
The classic pattern comes from the game Légal–Saint-Brie (Paris, c. 1750) and arises after 1.e4 e5 2.Nf3 d6 3.Bc4 Bg4 4.Nc3 Nc6?! 5.Nxe5!: Black has pinned White’s knight on f3 with the bishop on g4, but the pin is relative — the knight is pinned against the queen, not the king. White therefore sacrifices the queen with 5.Nxe5!, and if Black captures it (5...Bxd1), White plays 6.Bxf7+ Ke7 7.Nd5#. The Black king, lured to e7, is checkmated: the bishop on f7 gives check while the knight on d5 controls every escape square.
The key idea is the distinction between an absolute pin (against the king) and a relative pin (against a less valuable piece). In a relative pin, the pinned piece can move at the cost of a sacrifice, since the king is not directly exposed. It is precisely this breaking of the pin that makes the combination possible.
In practice, Legal’s mate is primarily an opening trap: it only works if the opponent captures the queen. If Black sees the danger and plays, for example, 5...Nxe5 or 5...dxe5 instead of taking the queen, White is simply a piece down with no compensation. Only attempt it when all the necessary conditions are in place.