That convenient little table of piece values that we all learn as beginners - pawn = 1, knight or bishop = 3, rook = 5, queen = 9 - can lead to a good deal of confusion. Particularly where material is unbalanced but nominally equal, these figures should be regarded as only an approximation. Piece values are far from absolute; they change as the game progresses.

Today's game offers an example of one of the most unequal fair exchanges, that of two minor pieces for a rook and a pawn or two. While a rook and a pawn may be a match for bishop and knight in an endgame (especially if the pawn is passed, and the minor pieces have no stable squares in the centre of the board), you generally need at least a rook and two pawns to match two minor pieces in the middlegame, for the simple reason that rooks need open lines if they are to function to their full effect.

In this game, White's combination just after the opening led to a position in which he had rook and three pawns for two pieces, but several of his moves then suggested that he was taken in by the fact that 5+3 is so much bigger than 3+3. White's 16.c5, 19.h4, 20.Bxc6 and 21.f3 were all poor moves, contriving to create a position in which White's pawn structure became too rigid. Instead, he should have been playing to open the game with f4, (as the game went, 19.f4! would have been much better for White) then get his centre pawns moving.

Black took advantage of all this to build up a K-side attack, culminating in 33...Nxg3! when 34.Rxg3 Rxh4+! 35.Kxh4 Qh6 is mate.

White: Simon Williams

Black: James Cobb

1 c4 e5 21 f3 Kh7

2 Nc3 Nf6 22 Kh2 Rg8

3 Nf3 Nc6 23 Qd2 Nh5

4 g3 Bc5 24 Rg1 f6

5 Bg2 d6 25 Rac1 Be8

6 d3 0-0 26 e3 f5

7 0-0 a6 27 d4 e4

8 Bg5 h6 28 f4 Nf6

9 Bh4 g5 29 Rc3 Bh5

10 Nxg5 hxg5 30 Kg2 Bf3+

11 Bxg5 d5 31 Kh3 Rg4

12 Nxd5 Be7 32 Ra3 Nh5

13 Nxe7+ Qxe7 33 Qe1 Nxg3

14 Qd2 Qd6 34 Qa5 Qh6

15 Bh6 Bf5 35 Qxc7+ Rg7

16 c5 Qe7 36 Qd8 Qg6

17 Qg5+ Bg6 37 Qg5 Qf7

18 Bxf8 Rxf8 38 Qd8 Qh5

19 h4 Qe6 39 Rc3 Qg4+

20 Bxc6 bxc6 40 Kh2 Nf1+

White resigned