Legal's Mate is essentially a trap in the Philidor's Defence. It was first recorded as being played by M. de Kermar and Sire de Legal (1702-1792). Legal played it against Saint Brie in 1750, his game played as the following: 3... Bg4 4. Bc4 g6 5. Nxe5 Bxd1 6. Bxf7+ Ke7 7. Nd5#.
In the main line White needs to be careful or else his Knight will be captured! In the decline line, White still does well gaining a Pawn. White's position is better, with his King in position to castle and a Knight developed. Legal's Mate is a must know for any Chess Player!