Another thing for which Ronald Knox is famous is the 'Ten Commandments' he proposed for the Detection Club:
I. 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.
II. All supernatural or preternatural agencies are ruled out as a matter of course.
III. No more than one secret room or passage is allowable.
IV. No hitherto undiscovered poisons may be used, nor any appliance which will need a long scientific explanation at the end.
V. No Chinaman must figure into the story.
VI. No accident must ever help the detective, nor must he ever have an unaccountable intuition which proves to be right.
VII. The detective must not, himself, commit the crime.
VIII. The detective must not light on any clues which are not instantly produced for the inspection of the reader.
IX. The stupid friend of the detective, the Watson, must not conceal any thoughts which pass through his mind; his intelligence must be slightly, but only very slightly, below that of the average reader.
X. Twin brothers, and doubles generally, must not appear unless we have been duly prepared for them.
The Chinaman rule is to prevent the villainous deed from being pawned off on mysterious foreigners. An interesting game is to think of mystery stories that are (1) competent; and (2) break at least one rule. Probably the most famous is Agatha Christie's The Murder of Roger Ackroyd. Chesterton (like a number of others) tends to play with the possibility of breaking rules without actually doing so, but at times he does produce a whoppingly good one that breaks one of the rules; one of my favorites along these lines is "Dr. Hyde, Detective, and the White Pillars Murder" -- which, indeed, is probably the best example of a good mystery story breaking the particular rule it breaks, although one can perhaps argue that it does so by arranging things so that it can both break the rule and follow it simultaneously.
