Only recently did I hear about Ronald Knox’s 10 Commandments of Detective Fiction, penned about a century ago. How much times have changed! Knox was undoubtedly attracted to the “10 Commandments” idea because he was a Catholic priest, but he also was a mystery writer who clubbed with notable mystery writers of his day. Below are his rules, and for many of them, I—and you, too—can think of entertaining exceptions! (Mine in parentheses.)

In contrast with the man pictured at left, who may feel his interpretation of the rules brooks no disagreement, we can see a lot of room for nuance here!

1. 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. (This leaves out the whole multiple point-of-view serial killer subgenre, but I will confess that, while multiple points of view don’t bother me, I don’t enjoy it when one of those viewpoints belongs to a predatory character. Creeps me out.)
2. All supernatural or preternatural agencies are ruled out as a matter of course. (So much for another whole category of thriller that agents and publishers today say they’re looking for, and books like Stephen King’s Mr. Mercedes. Someone forgot to tell Edgar Allan Poe, Bram Stoker, and Shirley Jackson about this, for three.)
3. Not more than one secret room or passage is allowable. (Hmm.)
4. No hitherto undiscovered poisons may be used, nor any appliance which will need a long scientific explanation at the end. (Agree. Sounds like cheating and very possibly boring.)
5. No Chinaman must figure in the story. (Yet one of my favorite detective quotes comes from Charlie Chan: “Theory like mist on eyeglasses. Obscures vision.” And this leaves out my all-time favorite detective, Judge Dee Goong An, in the novels by Robert Van Gulik. And S.J. Rozan!) The rule probably came about because a “Chinaman” had been overused as a sinister character, Fu Manchu moustache and all.
6. No accident must ever help the detective, nor must he (!) ever have an unaccountable intuition which proves to be right. (This rule lives on in objections to plot coincidences. They are annoying.)
7. The detective must not himself commit the crime. (Omitting a whole category of cops-gone-wrong and unreliable narrators. Tricky.)
8. The detective must not light on any clues which are not instantly produced for the inspection of the reader. (Did you ever have a “what just happened?” moment while reading? That’s sometimes because you didn’t see—or weren’t shown—the clue. Agree, it’s bad when this occurs.)
9. 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. (I object! Nigel Bruce’s characterization of Watson in the old movies was bumbling, but Watson is far from stupid. And what does that say about the “average reader”?)
10. Twin brothers (what about twin sisters, huh?), and doubles generally, must not appear unless we have been duly prepared for them. (Twins are a problem. But can move beyond clichéd mix-ups into unexpected territory, as in award-winning Japanese novelist Riku Onda’s fascinating mystery, Fish Swimming in Dappled Sunlight).