The Ten Commandments for Detective Fiction (1929): A Brief History and Update

Photo showing potential murder weapons (rope, knife, )and the post title'The 10 Commandments of Detective Fiction 1929) is superimposed on the top.

by Gretchen Mullen, @GretchenMdm9524

“Thou shalt not cheat thy reader”

Ronald Knox (1888-1957) was an English priest who moonlighted as a well-regarded author of detective novels and short stories. His reputation was such that in 1928, during the Golden Era of Detective Fiction, when a group of British mystery authors gathered to exchange ideas and collaborate, Knox was included in this elite group. Officially known as The Detection Club, the group formally organized in 1930. Membership was and still is by invitation only. Original members included such greats as Agatha Christie, Dorothy Sayers, and first elected president G.K. Chesterson.

Knox co-edited and penned the “Introduction” to The Best English Detective Stories of 1928. Knox’s essay (originally dated February 28, 1929), was later reprinted as “The Detective Story Decalogue” in 1946.

According to the Ronald Knox Society of North America, the Decalogue became known as “the Ten Commandments for Detective Novelists as a set of by-laws for the [Detection] club.” Often reprinted in short form, the commandments (also referred to as Rules of Fair Play) are meant to remind authors that the reader deserves a fighting chance to solve the mystery without the author’s use of cheap tricks.

While these commandments do not all hold up to today’s standards of political correctness or modern terminology, the essence of these nearly century-old rules remain remarkably salient. With that said, here are Knox’s Ten Commandments, as written in their original long form. Bold print indicates the short form of the Decalogue commonly reprinted, too often without attribution.

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. The mysterious stranger who turns up from nowhere in particular, from a ship as often as not, whose existence the reader had no means of suspecting from the outset, spoils the play altogether. The second half of the rule is more difficult to state precisely, especially in view of some remarkable performances by Mrs. Christie. It would be more exact to say that the author must not imply an attitude of mystification in the character who turns out to be the criminal.

II. All supernatural or preternatural agencies are ruled out as a matter of course. To solve a detective problem by such means would be like winning a race on the river by the use of a concealed motor – engine. And here I venture to think there is a limitation about Mr. Chesterton’s Father Brown stories. He nearly always tries to put us off the scent by suggesting that the crime must have been done by magic; and we know that he is too good a sportsman to fall back upon such a solution. Consequently, although we seldom guess the answer to his riddles, we usually miss the thrill of having suspected the wrong person.

III. Not more than one secret room or passage is allowable. I would add that a secret passage should not be brought in at all unless the action takes place in the kind of house where such devices might be expected. When I introduced one into a book myself, I was careful to point out beforehand that the house had belonged to Catholics in penal times. Mr. Milne’s secret passage in the Red House Mystery is hardly fair; if a modern house were so equipped – and it would be villainously expensive – all the countryside would be quite certain to know about it.

IV. No hitherto undiscovered poisons may be used, nor any appliance which will need a long scientific explanation at the end. There may be undiscovered poisons with quite unexpected reactions on the human system, but they have not been discovered yet, and until they are they must not be utilized in fiction; it is not cricket. Nearly all the cases of Dr. Thorndyke, as recorded by Mr. Austin Freeman, have the minor medical blemish; you have to go through a long science lecture at the end of the story in order to understand how clever the mystery was.

V. No Chinaman must figure in the story. Why this should be so I do not know, unless we can find a reason for it in our western habit of assuming that the Celestial is over-equipped in the matter of brains, and under-equipped in the matter of morals. I only offer it as a fact of observation that, if you are turning over the pages of a book and come across some mention of ‘the slit-like eyes of Chin Loo’, you had best put it down at once; it is bad. The only exception which occurs to my mind – there are probably others – is Lord Ernest Hamilton’s Four Tragedies of Memworth.

VI. No accident must ever help the detective, nor must he ever have an unaccountable intuition which proves to be right. That is perhaps too strongly stated; it is legitimate for the detective to have inspirations which he afterwards verifies, before he acts on them, by genuine investigation. And again, he will naturally have moments of clear vision, in which the bearings of the observations hitherto made will become suddenly evident to him. But he must not be allowed, for example, to look for the lost will in the works of the grandfather clock because an unaccountable instinct tells him that that is the right place to search. He must look there because he realizes that that is where he would have hidden it himself if he had been in the criminal’s place. And in general it should be observed that every detail of his thought – process, not merely the main outline of it, should be conscientiously audited when the explanation comes along at the end.

VII. The detective must not himself commit the crime. This applies only where the author personally vouches for the statement that the detective is a detective; a criminal may legitimately dress up as a detective, as in the Secret of Chimneys, and delude the other actors in the story with forged references.

VIII. The detective must not light on any clues which are not instantly produced for the inspection of the reader. Any writer can make a mystery by telling us that at this point the great Picklock Holes suddenly bent down and picked up from the ground an object which he refused to let his friend see. He whispers ‘Ha!’ and his face grows grave – all that is illegitimate mystery – making. The skill of the detective author consists in being able to produce his clues and flourish them defiantly in our faces: ‘There!’ he says, ‘what do you make of that?’ and we make nothing.

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 very slightly, below that of the average reader. This is a rule of perfection; it is not of the esse of the detective story to have a Watson at all. But if he does exist, he exists for the purpose of letting the reader have a sparring partner, as it were, against whom he can pit his brains. ‘I may have been a fool,’ he says to himself as he puts the book down, ‘but at least I wasn’t such a doddering fool as poor old Watson.’

X. Twin brothers, and doubles generally, must not appear unless we have been duly prepared for them. The dodge is too easy, and the supposition too improbable. I would add as a rider, that no criminal should be credited with exceptional powers of disguise unless we have had fair warning that he or she was accustomed to making up for the stage. How admirably is this indicated, for example, in Trent’s Last Case!

Detective Fiction in the 21st Century: Have the rules changed?

It would appear that one of the reasons Father Knox’s Ten Commandments are still so well-known today is the fact that modern readers continue to devour the classics—after all, Agatha Christie does maintain her place as the bestselling novelist of all time. However, consider the following suggested rules for today’s detective fiction author.

Thou shall not employ cartoonish or one-dimensional characters.

In 2009, author P.D. James, with a half-century of mystery writing under her belt, released Talking About Detective Fiction. James observed the biggest shift in detective fiction to be readers demanding more depth of character. James boldly criticized Christie for characters lacking psychological depth, preferring instead to emphasize the puzzle. Of Christie, James says, “Above all she is a literary conjuror who places her pasteboard characters face downwards and shuffles them with practiced cunning.” While plot and puzzle-solving remain paramount, these flat, hum-drum characters are no longer enough for today’s modern reader.

Thou shalt strive to create a detective who has flaws.

Contemporary award-winning author Mark Billingham asserts that today’s modern detective should be a bit less heroic and a bit more flawed, citing the enduring character of Sherlock Holmes, who exhibits both virtues and weaknesses. Billingham believes readers care more about a detective who is imperfect, and also appreciate the element of unpredictability this may provoke.

Thou shalt respect science and scientific realism.

Readers are more rational today and although they still love a nice escape from reality, they also demand reason. They want believable murders that make sense scientifically, that don’t occur in a fairytale-like artificial world. Logical  deduction and intelligence must prevail.

And above all (a reinforcement of Father Knox’s rules):

Thou shalt not cheat thy reader.

No matter how much time passes, the detective fiction fan continues to expect the mystery writer to play fair—that is, in order to solve the puzzle presented, the same clues the detective has must be shared with the reader. The reader must not be left feeling cheated or tricked. If this commandment is broken, expect the reader to throw the book across the room. If the book is being read digitally, there may be a less violent response, but only because the reader desires to protect the electronic device.

Do you have any other mystery writing commandments to share? 

Gretchen Mullen is a published author whose most recent release was coauthored with her mother. The Rushing Noise of Death: A Detective Flagg Mystery, is currently available in ebook format.





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11 thoughts on “The Ten Commandments for Detective Fiction (1929): A Brief History and Update

  1. No Chinaman? Sign of the times.
    I can think of a movie with twins that we don’t know about until the great revelation at the end – not a murder-mystery, but it was a mystery and there were murders. (Points if you can guess the film.)
    Don’t cheat the reader is a rule for all genres.

    1. Most definitely, and sadly, a sign of the times. Offensive language, stereotyping, and racism or anti-Semitism was a part of many vintage mysteries and gives us, from a historical perspective, a window on the day. Good to have moved forward! I like Gretchen’s modern spin on today’s rules much better. :)

    2. It is often speculated that this may be a reference to the evil Dr. Fu Manchu. Entire articles have been written about this one commandment alone!

      I’m thinking about the twin mystery… I’ll get back with my guess!

  2. It’s always really interesting to look at these rules, and how relevant they are or aren’t in today’s crime writing world. Thanks for the’ food for thought.’

    1. Margot, nice to know it made you think a bit–it certainly did that to me! Some writers of the day were so concerned with the fairness of clues that “cluefinders” indicating pages numbers where clues could be found where actually published!

  3. Hi Elizabeth and Gretchen – what a fascinating post … I guess the earlier books and authors were not as swamped with other ways of life… hence they used settings and travel as a way of character. I used to love the Father Brown stories and the Brother Cadfael ones … as too Christie … just fun and entertaining, as well as brilliantly full of red herrings – cheers to you both – Hilary

    1. Yes, Hilary, the closed setting seems to lend itself to a limited circle of suspects. Thankfully, they did not have to deal with technology issues and the traces it seems to leave behind.

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