The Legal sacrifice is a common pin-breaking sacrifice theme that is most often seen in the opening. It specifically occurs when the victim has not yet castled, and has pinned a knight at f3 or f6 by developing the queenside bishop to g4 or g5 respectively.
The most basic, and classic form of the Legal sacrifice occurs in Philidor's Defense, its first recorded mention being an 18th century game which was won by the French player de Legal, from whom the theme takes its name. The game went as follows []
1 e4 e5 2 Nf3 d6 3 Bc4 Bg4(?!)
This in and of itself is not totally unsound (it does have the merit of pinning a White knight that wants to support the useful central advance d4), although it is most likely not the best move - even with good play from here on, it is likely that Black will be forced to retreat this bishop, thus losing a tempo or else have to exchange it against White's f3 knight, giving White the advantage of two bishops and leaving Black himself with the dark-squared bishop that is something of a bad bishop in the current pawn structure.
4 Nc3 g6?
However, this move is a mistake that loses at least a pawn. See the diagrammed position. White is now in a position to forcibly break the pin on his f3 knight by making use of a checkmate threat. Note that almost any other unproductive move by Black such as 4 ... a6 or 4 ... h6 is a mistake for the same reason, on the other hand White's sacrifice can be easily prevented by most productive developing moves, e.g. 4 ... Nc6, 4 ... Nf6 or 4 ... Be7. This is a strong lesson on the importance of focusing on development in the opening rather than on unproductive pawn moves!
White wins at least a pawn. If Black retreats the attacked bishop, White does likewise with the e5 knight and Black is left with nothing for the captured pawn; if Black takes the knight (5 ... dxe5), the g4 bishop is unguarded and White plays 6 Qxg4, regaining the piece and again Black has lost a pawn. Finally, if Black takes the Queen, which is what Legal's opponent St. Brie actually did, here is what happens:
5 ... Bxd1 6 Bxf7+ Ke7 7 Nd5 mate!
Of what use is having won the lady?
1 e4 e5 2 Nf3 Nc6 3 Bb5 a6 4 Ba4 Nf6 5 Nc3 Bb4 6 Nd5
Threatening to win the b4 bishop by first exchanging 7 Bxc6, an example of removal of the guard. Therefore, Black retreats his bishop.
6 ... Ba5 7 O-O b5 8 Bb3 d6 9 d3 Bg4 10 c3 Ne7?
Unlike St. Brie's mistake, this move actually looks like a good idea. It begins to transfer the knight over to the king's wing (possibly to f4 via g6) and also prepares to force away White's well placed knight and challenge the center via c6 and d5. It's clear too that the classical Legal mate isn't possible, since Black's king can run to f8. Unfortunately for Chigorin, however, the obstruction of e7, combined with the fact that White can now break up Black's kingside pawn formation with Nxf6+ allows White to make the Legal sacrifice in a different and just as effective way:
In actual fact, 11 Nxf6+ first works just as well (11 ... gxf6 12 Nxe5! and if Black takes the knight, White takes the bishop, while if 12 ... Bxd1, we transpose into variation 1 below where Black gets mated)
Black has had a pawn snapped off, and once again the White queen is for the taking. What happens here if he takes it? Well, it's obvious that White continues with 12 Nxf6+, the only reasonable forcing move, and now Black has two choices:
1. 12 ... gxf6 leads to a mate: 13 Bxf7+ Kf8 14 Bh6 mate. 2. 12 ... Kf8 leads to a series of exchanges that leave White ahead on material nonetheless: 13 Nfd7+ (as a matter of fact, 13 Ned7+ works as well, as Black cannot then even move the King) Qxd7 (if Black moves the King instead, there follows 14 Bxf7 mate) 14 Nxd7+ Ke8 15 Rxd1 Kxd7, and while the inventory of major assets has been rebalanced, Black has nonetheless come out minus a pawn.
In actuality, Chigorin found the queen to not sit well in his stomach and unlike St. Brie chose to take the knight instead. There thus followed:
11 ... dxe5 12 Nxf6+ gxf6 13 Qxg4
and just as in variation 2 above, Black has come out a pawn to the bad. In this case it can be regained by 13 ... Qxd3, but there then follows 14 Rd1 and Black's game collapses (for instance, if he tries to save the queen via 14 ... h5, White has 15 Qh4 Ng6 16 Qxf6 and still wins the queen because 16 ... Qxe4 allows 17 Qxf7 mate).
Of course, neither the closed openings nor White are entirely immune to the Legal sacrifice either. A short story comes from the Budapest Defense:
1 d4 Nf6 2 c4 e5 3 d5 Bc5 4 Bg5??
While 3 d5 is unpopular compared to accepting Black's gambit, it is nonetheless sound. This move, however, is a fatal blunder that costs White much material, at the very least.
4 ... Ne4!!
and once again the pin is rendered invalid due to a mate threat (if 5 Bxd8 Bxf2 mate!). White can only save everything for the moment by
but Black still forces a decisive advantage anyway by:
5 ... Bxe3 6 fxe3 Qh4+ 7 g3 Nxg3! 8 Nf3
The only way to simultaneously save the rook and prevent 8 ... Ne4 mate.
8 ... Qh6 9 Rg1 Nf5
and White will lose a second pawn, in addition to having a hopelessly shattered position.
As a general rule, it is seen that the Legal sacrifice works because of the weakness of KB2 in an uncastled position. Against a castled king, such maneuvers are much rarer and the relative pin tends to be far more stable. Thus, the Legal sacrifice represents another lesson in the importance of castling early in the game (as we saw from the 1893 game, even top flight masters of their days have been known to come a cropper to this theme).