A pin occurs when a piece is immobilized by an attack in such a way that, if the attacked piece moves, a more valuable piece behind it is lost. Pins are demonstrated in the following games.
Bronstein vs. Amateur (Simultaneous Exhibition, 1950) 1. e4 e5 2. d4 exd4 3. Qxd4 Nc6 4. Qa4 Nf6 5. Nc3 d5 The black knight on c6 is now pinned against the king, because if the black knight moves the king is lost to the white queen. This is called an absolute pin because it is illegal for the black knight to move, as such a move would place the black king in check. Notice that black stepped into this pin voluntarily by moving the d-pawn. 6. Bg5 And now black's f6 knight is also pinned by the white bishop. In this case, the knight is pinned to the queen. It is not illegal for the knight to move, just unwise as the queen would then be lost to the bishop. Such a pin is called a relative pin because the pinned piece can move legally. 6...dxe4 7. Nxe4 White attacks the pinned knight with a second piece. This is one way of exploiting a pin. 7...Qe7 Black defends by pinning the attacking piece, the white knight on e4. The white knight is immobilized by this absolute pin. 8. O-O-O Has white lost his mind? Can't the e4 knight now be taken? Watch the power of the pin! 8...Qxe4 Black captures a piece and unpins his f6 knight. The f6 knight now protects the queen against 9. Qxe4. Also, 9. Bxf6 meets with 9...Qxa4. What is white to do? 9. Rd8+! White forces the king to a square behind the f6 knight, either by 9...Kxd8 or 9...Ke7. As the f6 knight will then be pinned absolutely, black's queen will be left without protection. E.g., 9...Kxd8 10. Qxe4 and the white queen cannot be recaptured because the black knight is pinned. Black resigned at this point. He might also have tried to break the pin on his knight by 10 ... Be7. A pin can often be broken by putting a piece between the pinned piece and the piece it is pinned to. Black is not all that far behind in material, but the surprising lose of his queen no doubt took a psychological toll.
Pins do not always work as planned. Witness the violent unpinning that occurs in Legall's mate. 1. e4 e5 2. Nf3 Nc6 3. Bc4 d6 4. Nc3 Bg4? It would seem the white knight is now pinned against the white queen and cannot move without costing the queen her life. But this is a relative pin, and relative pins can be tricky. 5. Nxe5? Bxd1?? 6. Bxf7+ Ke7 7. Nd5++. If black had only looked more closely, he might have played 5...Nxe5. He would then have had a winning material advantage, which is why white's 5th move, although tricky, is bad. It is always unwise (and also bad chess) to make a questionable move in the hope that your opponent will miss something. Always assume your opponent sees everything you do.
Another example where a pin did not work as expected occurs in this game fragment from Tarrasch vs. Meiser (1890). This is an odds game, in which white gave odds of a knight. Remove white's queen's knight before playing through the game. 1. e4 d5 2. e5 d4 3. f4 c5 4. Bc4 Nc6 5. Nf3 Bg4? Seemingly, the white knight is pinned. In fact, this move loses a pawn. 6. Bxf7+ Kxf7 7. Ng5+ Ke8 8. Qxg4. This ends the lesson on failed pins from this game, but the game, or rather the commentary on it, holds another lesson. 8...Nh6 9. Qh3 Qd7 10. Qb3 Na5 11. e6 Qc6 12. Qh3 Rd8. Reuben Fine, one of the world's strongest players in the 1940s, commented on this game in his book, The Middle Game in Chess, as follows: Black's last move is, "A senseless, pointless move. Why does black drift into a hopeless position? Because he fails to develop properly. Correct was 12...g6 followed by ...Bg7 and ...O-O. Black could also castle Q-side after 12...Qd5." What is wrong with Dr. Fine's analysis? [Answer: the black king has already moved, making castling illegal.]
Here is another game in which a pin played a crucial role. It is Alekhine vs. four allies (1935). 1. e4 c6 2. d4 d5 3. Nc3 dxe4 4. Nxe4 Nd7 5. Qe2 Ngf6?? 6. Nd6++. Why can't black capture the white knight? Because the pawn on e7, which should be defending the vital d6 square, is pinned to its king and can't move. A pinned piece defends nothing, no matter what the appearances are.
Here is another game between two of the strongest players in the world, Tarrasch vs. Bogoljubov (Sweden, 1920). 1. d4 Nf6 2. c4 e6 3. Nf3 White can also play 3. Nc3, but black then commonly plays 3...Bb4 pinning the white knight. 3...b6 4. Bg5 Pinning the black knight. Watch how black breaks this pin. 4...Bb7 5. e3 h6 6. Bh4 Bb4+ 7. Nbd2 The white knight on d2 is now pinned, fatally as it turns out. 7...g5 Breaking the pin on the f6 knight. Normally, these pawn moves would be too weakening to the kingside pawn structure, but black has something else in mind. 8. Bg3 g4 Attacking a defender of the pinned d2 knight and forcing it to move. 9. Ne5 Ne4 and white resigned. The pinned knight is attacked a second time and is lost! Black has gained a winning material advantage by pouncing on a pinned piece.
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