chess

This problem, composed in 1959 by N Petrovic, led to a change in the laws. It is White to play and mate in 8 moves.

Barging in with 1.Qf7+ Kd8 2.Ne6+ looks attractive, but after 2...Kc8! Black's king slinks away and holds out beyond the eighth move.

The next idea is 1.Qc3, threatening both Qxh8 mate and Qf6 followed by Qe7 mate, but that lets Black slip away with 1...0-0 or 1...0-0-0.

Going back to the first line, we might get the idea of 1.Bd3, threatening 2.Qf7+ Kd8 3.Ne6+ Kc8 4.Bxa6+, but Black can meet that by driving the bishop back to b1 with 1...Rh1+.

The penny should now begin to drop: after ...Rh1+, Black can no longer castle K-side. All we have to do now is stop him castling long.

The solution is 1.Qb7! Rd8 (1...0-0 2.Qxd7 leads to a quick mate) 2.Qb3! Ra8 (to give the king an escape after Qf7+) 3.Bd3! Rh1+ 4.Bb1 Rh8 (to stop Qg8 mate) and now 5.Qc3! Rh7 (best) 6.Qf6 Rg7 (otherwise Nxh7 follows) 7.Qxg7 and mate next move.

Under the old rules, Black could claim a draw by threefold repetition after 4...Rh8. Now one must have not only identical positions, but identical possibilities, including en passant captures and castling.