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The term "Fairy Chess" was coined by the brilliant British problemist Thomas Rayner Dawson to cover all deviant versions of the game. Chess on a cylinder, or a torus, or where the object is to lose all your pieces, or where pieces move in unorthodox ways - all are part of the Fairy family.

Today's problem, by Dawson himself, is a splendid Fairy miniature. The task is to add a black bishop and a black rook to produce an "illegal cluster". It's easy enough to produce an illegal position - just put the pieces anywhere so that both deliver check - but an illegal cluster is precisely defined. To fulfil the objective, not only must the position be illegal, but removing any piece, other than a king, removes the illegality.

So, for example, if we put the rook on h4 and bishop on b6, we can remove the rook (when Black has just played Nc5-e6+ or the bishop (when the last move was Nf4-e6+) but taking the knight away leaves a still illegal position. Stop reading now and try to find the answer.

Given up already? Try Rg4, and Bh8. Minus the bishop, the last move was Nf4-e6, minus rook it's Ng7-e6; minus knight it's Rg7-g4. And that's the only way to do it.