"1. The criminal 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 supernaural or preternatural agencies are ruled out as a matter of course.
3. Not more than one secret room or passage is allowable.
4. No hitherto undiscovered poisons may be used, nor any appliance which will need a long scientific explanation at the end.
6. No accident must ever help the detective, nor must he ever have an unaccountable intuition which proves to be right.
7. The detective must not himself commit the crime.
8. The detective must not light on any clues which are 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 brothers, and doubles generally, must not appear unless we have been duly prepared for them."
Father Knox’s ten rules for writing a mystery, 1929. (The racist one is taken out)
(Source: the-right-writing, via clevergirlhelps)