In 1928, theologian and mystery writer Ronald Knox codified 10 rules of detective fiction:
- The criminal must be mentioned in the early part of the story, but must not be anyone whose thoughts the reader has been allowed to know.
- All supernatural or preternatural agencies are ruled out as a matter of course.
- Not more than one secret room or passage is allowable.
- No hitherto undiscovered poisons may be used, nor any appliance which will need a long scientific explanation at the end.
- No Chinaman must figure in the story.
See full list at http://www.futilitycloset.com/2012/03/29/ten-commandments/