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Nowhere is the inherent logic of chess better exploited than in the inherently illogical problemist's art of retro-analysis, where the solver is asked not to find the best move in a given position but to work out its past history. As these example s show, such a history may date back to the beginning of time.

Composed by Markus Ott in 1982, this is the position after White's 19th move. What were the moves?

gvdxacf, nhnhn n , C n n , , , , G,H,S,H, ,D, ,F, HN ZHN N ,G, ,F, White's formation requires five bishop moves, three knight moves, two with the queen, two with pawns and one with the king. The rooks need at least five moves when you take into account necessary blockages from other men. One more move is needed to capture the missing black rook, which has to die on g8 or h7. That accounts for all 19 White moves.

Black's are trickier: a white pawn had to be captured on d2. The only man that could have done so is a knight which, in making the round, must take an even number of moves. The rook, in order to give its life on g8 or h7, must make an odd number of moves. Adding two black pawn moves leaves us with a parity problem to reach 18 black moves. The only solution is that the king has been wandering and lost a move.

That's enough logic. We can now skip to the solution: 1.g4 Nf6 2.Bg2 Rg8! 3.Bd5 Ne4 4.Nf3 f6 5.Bxg8 Nxd2 6.Bb3 Nc4 7.Bf4 Kf7! 8.Bd6 Kg8 9.Qd5+ Kh8! 10.Nbd2 Ne5 11.0-0-0!! Nf7 12.Rhe1! Nh6 13.Nf1 Ng8 14.Rd4 h6 15.Ra4 Kh7 16.c4 Kg6 17.Qe4+ Kf7 18.Kd2 Ke8 19.Rb1.

If you think that was ingenious, try the record-holder, composed by M. Caillaud: f, , Bf, b nhnh,f fn , n , , , , , b B N , NHZ , , d,HNHN , zD, , c This is the position after Black's 47th move and there is only one way to have got there. It begins 1.Nf3a5 2.Nd4 a4 3.Nb3 axb3 4.a3 Ra4 5.Ra2 bxa2 6.Rg1 axb1=B! and I shall never tell you the rest. Sometimes it's better just to boggle.

William Hartston