Fair-Play Mystery

A fair-play mystery is one bound by the convention that a detective story must give its audience every clue the detective uses, so that a careful reader could, in principle, solve the case before the reveal.

The convention was codified rather than merely observed. Ronald Knox's 1929 "decalogue" and S. S. Van Dine's twenty rules, both published as prescriptions for the genre, ban devices that make fair solving impossible — an unintroduced twin, a culprit who never appears until the reveal, a vital clue withheld from the reader though visible to the detective. Van Dine went further and tried to ban entire subgenres, a reminder that these were house rules for a moment in the genre, not timeless law.

The closed circle — a fixed, enumerated set of suspects, isolated by a country house, a snowed-in train, an island — makes fair play tractable: the audience needs a bounded who and how before deduction is possible, and unlimited suspects make the contract unenforceable, a Legibility requirement as much as a narrative one.

Interactive form changes the convention's enforcement from etiquette to mechanism. A novel can only be judged fair play after the fact, by a reader checking the ending against the clues; a game with an inventory or a "confront" action often cannot present the accusation screen until specific evidence has been collected — Schema Validation applied to plot logic, checked at the point of action rather than after. It converts a courtesy the author owed the reader into a rule the software enforces, the same contract Chekhov's Gun runs on, tightened until it depends on the same Diegesis question every mystery answers: is a clue in the world, or only in the text describing it?