Robert Knox’s 10 Commandments of Detective Fiction

Are you familiar with Robert Knox’s 10 Commandments of Detective Fiction? I wasn’t until recently. But if you want to write a mystery you might want to acquaint yourself with them.

And then feel free to break them.

I read and write a lot of thrillers. I’ve tried my had at writing ghost stories. In fact, my first commercially published piece is a ghost story published in the anthology Fireside: Modern Legends and Lore. But recently I’ve been obsessed with all thing mystery.

From Anthony Horowitz to Louise Penny, I can’t get enough of this genre. But I have a pretty high standard for these stories. They have to pass a series of [what I thought were] unwritten rules for me to feel satisfied at the end. So when I stumbled upon Knox’s 10 Commandments of Detective Fiction (in the novel Everyone in My family Has Killed Someone – read the review here) I felt validated. It wasn’t just me who wanted these stories to abide by some universal rules.

Richard Knox

Ronald Knox was a mystery writer in the early 20^th century. he was a member of the Detection Club, with legendary mystery writers like Agatha Christie, Dorothy Sayers, G. K. Chesterson, and E. C. Bentley. He wrote several classic British mysteries including, The Viaduct Murder, Still Dead, and The Body in the Silo. 

Knox was also a Catholic priest. Perhaps that’s why he was tempted to write a 10 Commandments of detective fiction. If you write such stories, thou shalt obey these laws.

Let’s take a look at them:

The 10 Commandments of Detective Fiction (Golden Age)

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

Do the 10 Commandments of Detective Fiction still apply?

In addition to being a priest, Knox was also a mystery writer, humorist, and satirist. So presumably even he did not believe that authors were bound by these rules.

And certainly most modern psychological thrillers bend and break these rules at will. Unreliable narrators, long-lost twin brothers, convenient accidents are all alive and well in many thrillers today.

But I do believe we can learn something from these rules. Well, except for number 5. That’s just racist and abhorrent and without need for further discussion. Just strike that one from the list.

Rules 1 & 7

Rule 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.”

Rule 7: “The detective himself must not commit the crime.”

Modern thrillers routinely ignore these rules. Gone Girl is perhaps the most notorious version of the unreliable narrator. But there are lots of other examples: The Girl on the Train, In a Dark, Dark Wood, even One of Us is Lying. We often get the inside scoop on what the criminal is thinking from the get go. But can you believe them?

While this type os story is becoming a bit overdone, when done well the unreliable narrator makes for a great plot twist. And it’s still one of my favorite types of stories. So go ahed and break these rules. Just be careful doing it. And be prepared to classify your books as a thriller and not a mystery.

Rule 2

Rule number 2 states that “All supernatural or preternatural agencies are ruled out as a matter of course.” The occult would have been particularly objectionable to Knox– priest-who viewed the supernatural as evidence of the loss of faith in God. Still, even without any religious connotations, I think we can all agree that paranormal explanations have no place in a mystery. If you want to write about ghosts and supernatural powers, by all means go ahead. Just call it a ghost story or speculative fiction.

Rule 3, 4, 6 and 10.

Rule 3: “Not more than one secret room or passage is allowable.”

Rule 4: “No hitherto undiscovered poisons may be used, nor any appliance which will need a long scientific explanation at the end.”

Rule 6: “No accident must ever help the detective, nor must he ever have an unaccountable intuition which proves to be right.”

Rule 10: “Twin brothers, and doubles generally, must not appear unless we have been duly prepared for them.”

These rules go to plausibility. You want your reader to buy into your story. And you don’t want unbelievable plot holes distracting them while they are reading. As a trained lawyer, having a cohesive story neatly tied up with a bow, is very important to me. And I think it’s just as important in books as it is in court.

As a book coach I am always pushing my writers on fortuitous encounters, happy accidents, and coincidences. “There’s no such thing as a coincidence,” I like to tell them.

You can include twins and look alikes, half-siblings, long-lost family members if you want to. But I would do so sparingly. And definitely intentionally. It’s really obvious when this sort of “plot twist” this used in place of good writing or story development. Lay the groundwork from the get-go. When it comes to the reveal your reader should be nodding their head in agreement, not saying, “Wait, what?”

The same thing goes with intuition. Sure most sleuths have some level of intuition. It helps make them good at their work. But you should be alluding to this character trait throughout your story. Don’t just bring it up when there is no logical way to draw a conclusion. Evert solution must be used in fact and your reader needs to see those facts on the page.

As for rule number 4 about poisons and scientific explanations, I am pretty much a stickler for this one. I want need my story to make sense This means that the method of murder needs to be realistic. I will spend hours (and hours and hours) researching details to get it right. But it also needs to be straightforward enough that the lay person can understand it. The reader does not want to get into a deep scientific analysis of how to compounds react in the body. Don’t make the murder method any more complicated than you need to.

And if you want to introduce imaginary poisons or farfetched causes of death, go ahead. Just right a fantasy.

I don’t much care about hidden passageways now way to another. I’d love to live in a home with one some day. As far as a writing crutch, I don’t see this overdone too much. It is totally appropriate in a classic mystery – very Clue-esque. But be careful how many you include. Again, if you’re relying to such on secret rooms you probably need to go back an rework you story outline. Or maybe you are writing fantasy.

Rules 8 & 9

Finally, let’s talk about the last two rules:

Rule 8: “The detective must not light on any clues which are not instantly produced for the inspection of the reader.”

And Rule 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.”

One important rule of good writing is that you don’t want insult your reader’s intelligence. These two rules are designed to keep you reader engaged. Although I would argue with the word “instantly” in rule 8.

But think about it for a second. If you get to the end of the story and your sleuth solves the crime by relying on clues that you have never disclosed to reader they are going to pissed. They are going to say the story didn’t make sense. They are going to feel hoodwinked. None of these are good for reader satisfaction or recommendations. So be sure to share all the important clues with your reader during the story. You want to give then the opportunity to solve the crime on their own.

The Watson rule is also good advice, if you include a “stupid friend” in your mystery. During the classic age of mysteries, the Watson character was often used to help the detective puzzle out the mystery and explain things to the reader. This character also reassures the reader that they are not too far off base with their theories because even a character in the story had similar thoughts. Keeping this character slightly below the average intelligence of your reader ensures you won’t piss off readers who are made to feel dumb by a high-brow mystery or complicated plot twists. This can still be an important literary tool, but is not crucial to a successful mystery story.

How to use the 10 Commandments of Detective Fiction

So what do you think? Are the 10 Commandments of Detective Fiction still helpful today?

I think they can serve as a good reminder to make sure you are telling a believable story that makes sense to readers. But I don’t think you have to adhere to all of them all of the time.

But you know what they say, “You’ve got to know the rules, before you can break the rules.” And maybe just don’t break all of them at the same time.

If you’d like to learn more about how to set up a great mystery, contact me. Or check out my book coaching packages.

Happy Writing!