If one reads about the golden age of detective fiction (roughly, the inter-war period, circa 1919-1939), one is apt to come across some of the formulations of rules for detective fiction written then. As I’ve noted, detective fiction has from its inception been a self-referential genre, and it was talked about even more outside of the pages of the mystery novel than inside them.

One of the most famous lists of rules is the Decalogue (ten commandments) set down by Fr. Ronald Knox. They are:

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 must not light on any clues are not instantly produced for the inspection of the reader.
9. 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.
10. Twin brothers, and doubles generally, must not appear unless we have been duly prepared for them.

I propose to go through each of these in one post per rule (which I will eventually link here) and discuss them. In aid whereof, this page which has some of Fr. Knox’s own commentary on his rules (years after he wrote them) will be especially interesting.

I should note that in general I agree with the Decalogue. My intention is not criticism but consideration—to look at the purpose of the rule and as such when it can be broken in good faith.

• Commandment #1: Unintroduced Murderers
• Commandment #2: The Supernatural
• Commandment #3: Secret Passages
• Commandment #4: Poisons and Devices
• Commandment #5: Chinamen
• Commandment #6: Accidents
• Commandment #7: Detective Murderers
• Commandment #8: Clues
• Commandment #9: The Watson
• Commandment #10: Disguises