Chess: The case of Moriarty's last move

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The Independent Culture
'HURRY, Watson] There's no time to lose.'

Rapidly putting down his violin, pipe, hypodermic syringe and presentation pack of stamps from the Royal Mail, Sherlock Holmes rushed out of the door of number 221b Baker Street, pursued by a panting Dr Watson.

'What's the hurry, my friend?' asked Watson. 'To judge from your urgency, I'll wager it's something to do with that fiend Moriarty.'

'I applaud your logic, Watson,' said the great detective. 'I have just heard that he is on the verge of winning the championship of the Marylebone Chess Club. We must stop him before it is too late.'

The pair raced up the stairs of a depressing grey-stoned building. As they entered the premises of the chess club, everyone was gathered round a single board, on which the pieces stood as in the appended diagram. But of Professor Moriarty there was no sign.

'We're too late,' sighed Watson.

'And how do you reach that conclusion?' asked Holmes, with a twinkle in his voice.

'White's mated,' said Watson. 'Moriarty's won the game and the championship.' He moved his hand towards the pieces to begin putting them back into the box.

'Don't move those chessmen,' shouted Holmes. 'For they are the proof that Moriarty is finally defeated.'

'Nonsense, Holmes. Moriarty was White, not Black. It's checkmate.'

'But what was Black's last move?' Holmes asked.

'Pawn to b5 mate,' said Watson. 'Ah, I see what you're getting at. If we can prove that the pawn came from b7, then White can take en passant and . . . my goodness, Holmes, it's Black who's mated. But Moriarty has taken the scoresheets. We'll never prove without them that the pawn did indeed come from b7.'

'Think again, Watson,' Holmes said. 'What was White's last move? Not, for sure, with the rook on a1, nor the bishop or knight, for neither can have moved without Black's king having been illegally in check. The white pawns also have no possible previous move.'

'So it's king or rook,' mused Watson.

'But look at the king,' continued Holmes. 'Any square it could conceivably have come from, it would have been in a check which could not have been realised by legal play.'

Watson stared at the board in disbelief.

'And so,' continued Holmes, 'White's last move must have been with his rook, from b6 to a6.'

'And Black's last move, therefore,' added Watson, 'cannot have been from b6 to b5.'

'Bravo,' said Holmes. He reached for the pawn on a5, placed it firmly on b6, removed the pawn from b5 and said: 'Checkmate, Professor Moriarty.'