School of Hard Knox

If the name of Ronald Knox, the author of The Footsteps at the Lock, is familiar to you, it is most probably because of the ten rules he proposed for writing detective stories. As he was also a Catholic clergyman, Monsignor Knox not too surprisingly called his ten rules Ten Commandments for Detective Fiction - a Decalogue, if you will. The rules (as listed in Wikipedia):

1. 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.
2. All supernatural or preternatural agencies are ruled out as a matter of course.
3. Not more than one secret room or passage is allowable.
4. No hitherto undiscovered poisons may be used, nor any appliance which will need a long scientific explanation at the end.
5. No Chinaman must figure in the story.
6. No accident must ever help the detective, nor must he ever have an unaccountable intuition which proves to be right.
7. The detective himself must not commit the crime.
8. The detective is bound to declare any clues which he may discover.
9. The "sidekick" 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.
10. Twin brothers, and doubles generally, must not appear unless we have been duly prepared for them.

The astute reader will undoubtedly notice that each and every one of these "rules" has been violated with impunity by just about every great writer of detective fiction, so it's worth pointing out, as Martin Edwards does in his discussion of Knox in The Golden Age of Murder, that Knox did not take his own rules very seriously, noting that "Rules so numerous and so stringent cannot fail to cramp the style of the author." Still, for those of us who approach these books looking for some degree of fair play in the ways an author can misdirect us, the rules of the game certainly are worth knowing.