Ronald Knox: 10 Commandments of Detective Fiction
  1. criminal must be mentioned in the early part of the story, but not someone the reader can follow their thoughts
  2. supernatural and prenatural agencies are ruled out as a matter of course
  3. not more than one secrete room of passage is allowable
  4. no undiscovered poisons or any appliances that require long scientific explanations
  5. no chinaman must figure in the story (???)
  6. no accidents can help the detective, nor unaccountable intuitions be proved correct
  7. detective himself cannot 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’, must not conceal any thoughts which pass through their mind; their intelligence must be slightly below that of the average reader
  10. twin brothers and doubles must not appear unless they’ve been duly prepared for them
S.S. Van Dine: 20 Rules for Writing Detective Stories
  1. reader must have an equal oppurtunity to solve the case as the detective
  2. no wilful tricks may be played on the reader other than those legitimized by the detective or criminal themselves
  3. no love interest in the story
  4. the detective should not turn out the culprit
  5. culprit must be determined by logical deductions
  6. detective novels must have a detective that plays the functional role of the detective
  7. there must be a corpse; the deader the corpse the better
  8. problem of the crime must be solved through naturalistic means
  9. there must only be one detective
  10. culprit must have been someone that played somewhat of a prominent role in the story
  11. servants must not be chosen as the culprit by the author
  12. there must only be one culprit, but may have minor helpers or co-plotters
  13. secret societies have no place in a detective novel
  14. method/means of murder must be rational and scientific
  15. the truth of the problem must at all times be apparent; if re-read, the reader should be able to point out the clues that lead to the culprit
  16. should not contain any long descriptive passages
  17. professional criminals shouldn’t be shouldered with the guilt of crime
  18. a crime should never turn out to be an accident or a suicide
  19. the motives for all crimes should be personal
  20. avoid following situations as they’ve been overused and overdone…