In the late 1920s, the precocious classicist, theologian and detective novelist Ronald Knox (whose biography was written by Evelyn Waugh) set down 10 Rules For Detective Fiction. I'm not sure how many of them apply in the more contemporary and modish school of Scandinavian crime writing, and no.5 might be considered problematic in any genre today, but here they are:
- 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.
- No accident must ever help the detective, nor must he ever have an unaccountable intuition which proves to be right.
- The detective himself must not commit the crime.
- The detective is bound to declare any clues which he may discover.
- The stupid friend of the detective, the Watson, must not conceal from the reader any thoughts which pass through his mind: his intelligence must be slightly, but very slightly, below that of the average reader.
- Twin brothers, and doubles generally, must not appear unless we have been duly prepared for them.
[Reprinted in Haycraft, Howard, Murder for Pleasure: The Life and Times of the Detective Story, Revised edition, New York: Biblio & Tannen, 1976.]