A chess position is frozen on the board. Can you prove how it was reached?
Retrograde analysis chess puzzles give you a static position and ask not for the best next move, but to logically deduce the history that led to it: which piece must have just moved, whether castling is still legal, or whether a pawn promoted from an unlikely square. Raymond Smullyan built two acclaimed puzzle books entirely from this genre, framed as Sherlock Holmes-style mysteries. What single fact about the position would prove a 'legal' history exists at all?
Reveal the answer
A position only needs to be reachable by some legal sequence of moves, however strange, not a sensible one; solvers work backward from the board, ruling out piece arrangements no legal game could ever produce. Raymond Smullyan's 'The Chess Mysteries of Sherlock Holmes' (1979) and 'The Chess Mysteries of the Arabian Knights' (1981) turned this deductive technique, previously a niche chess-composition tool, into puzzle books built entirely around solving whodunit-style mysteries using nothing but the rules of chess.
— Raymond Smullyan, The Chess Mysteries of Sherlock Holmes — Alfred A. Knopf, 1979
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