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In 1928, the writer Father Ronald Knox created a "Ten Commandments" of plot devices that more or less codified the rules of the Fair-play whodunnit:
#:1. The criminal [[Chekhov's Gunman|must be someone mentioned in the early part of the story]], but must not be anyone whose thoughts the reader has been allowed to follow.
#:2. All supernatural or preternatural agencies are ruled out as a matter of course.
#:3. Not more than one secret room or passage is allowable, and such a passage may only be in a house or building for which it is appropriate by age or purpose.
#:4. No hitherto undiscovered poisons may be used, nor any appliance which will need [[Techno Babble|a long scientific explanation at the end]].
#:5. No Chinaman must figure in the story.
*::* Note that this was not a case of racism (though the now-offensive but generally obsolete term "Chinaman" is a case of [[Values Dissonance]]), but a reaction against the clichéd [[Yellow Peril]] villains prevalent in dodgy crime fiction at the time. The modern American equivalent would also include a Middle Eastern character or a [[Scary Black Man]], or [[Narrowed It Down to the Guy I Recognize|anyone known to have enough acting ability to pull off the role]].
#:6. No accident must ever [[Deus Ex Machina|help the detective]], nor must he ever have [[I Just Knew|an unaccountable intuition]] [[Bat Deduction|which proves to be right]].
#:7. [[Detective Mole|The detective must not himself commit the crime.]]
#:8. The detective must not light on any clues which are [[Clueless Mystery|not instantly produced]] for the inspection of the reader.
#:9. The stupid friend of the detective, the "Watson", must not conceal any thoughts which pass through his mind; his intelligence must be slightly, but very slightly, below that of the average reader.
#:10. [[Twin Switch|Twin brothers]], and [[Body Double|doubles]] generally, must not appear unless we have been duly prepared for them.
[http://www.mysterylist.com/declog.htm Other such lists can be found here.]
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