In retrospect, the earliest days of mystery fiction feel much like the Wild West — anything is fair game. Victorian-era authors relied on unbelievable coincidences, absurd jumps in logic, and outright chicanery to wrap up their stories. Some 30 years later, a sheriff brought order this lawless land. Anglican priest and prolific author Ronald Knox suggested a common set of rules to which mystery authors should adhere.

In Knox’s proposal, the murder presents as a puzzle. This game plays with all its pieces and players left on the table. Nothing appears hidden, and the solution remains in plain sight, if one only knows where to look. As he wrote, all detective fiction must focus on “the unraveling of a mystery; a mystery whose elements are clearly presented to the reader at an early stage in the proceedings, and whose nature is such as to arouse curiosity.”

He also noted that this curiosity is one which must be gratified at the end. With the help of a brilliant detective—police, consulting, amateur or otherwise, the reader embarks on a trip. They should journey through shocking crime scenes, past tempting red herrings and, at last, to a satisfying solution. That satisfaction hinges, in large part, on the understanding that the game plays fairly. In 1929, Knox codified his rules of detective fiction into a concise list of ten commandments.

Knox’s Decalogue

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 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.

These rules of the game served as guiding principles for dozens of authors who populated murder’s Golden Age. Most mystery aficionados are familiar with names like Agatha Christie, Dorothy L. Sayers, John Dickson Carr and Ellery Queen. Their work still thrives today, and their trademarks — from amateur sleuths to impossible locked room mysteries — form the foundation of diverse modern genres from cozies to political thrillers.

The best of these authors — Christie, Carr, and Queen in particular — mastered the art of bending (without ever breaking) Knox’s rules. For example, Christie first made a major splash with The Murder of Roger Ackroyd. At the time, the solution appeared revolutionary, almost outrageous. After careful review, critics noted that Christie had played by the rules. She simply tailored the game to suit her own tastes.

So, which of these rules are most critical to you as a reader? Which Golden Age or modern mystery authors do you consider guilty of breaking these commandments? I’d love to hear your thoughts!

Love a good murder? Be sure to take a stab at Thou Shalt Not Kilt, a traditional Southern whodunnit with Scottish flavor. For my latest news and updates, follow me on Instagram and TikTok. If you love crime fiction, sign up for Fatal Fiction, my monthly mystery newsletter. You’ll get a free download of Masters of Murder, my concise guide to the authors of mystery’s Golden Age.