Knox's Commandments

According to Father Ronald A. Knox, these 10 rules applied to Golden Age detective fiction:

#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.
#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 must not himself commit the crime.
#The detective must not light on any clues which are not instantly produced for the inspection of the reader.
#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.
#Twin brothers, and doubles generally, must not appear unless we have been duly prepared for them.

Compare the "Twenty Rules for Writing Detective Stories" by S. S. Van Dine. [http://www.sfu.ca/english/Gillies/Engl38301/rules.htm]

See also Detection Club.