Gesetze – Ronald Knox

Ronald Knox: Ten Commandment List for Detective Novelists (1929)

  • 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.   

  • All supernatural or preternatural agencies are ruled out as a matter of course.   

  • Not more than one secret room or passage is allowable.   

  • No hitherto undiscovered poisons may be used, nor any appliance which will need a long scientific explanation at the end.   

  • No Chinaman must figure in the story.   

  • No accident must ever help the detective, nor must he ever have an unaccountable intuition which proves to be right.   

  • The detective must not himself commit the crime.   

  • The detective must not light on any clues which are not instantly produced for the inspection of the reader.   

  • 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.   

  • Twin brothers, and doubles generally, must not appear unless we have been duly prepared for them.   

Grobius Shortling’s Revised Version of Ronald Knox’ Decalogue (2001)


  • The criminal must be somebody mentioned in the story. (This is absolutely essential, otherwise the book cannot be called a detective story. The other bit about ‘sharing thoughts’ is too strict, but a writer should still be cautious because an outright authorial deception must be avoided.)   

  • Supernatural elements are allowable for atmospheric or plot reasons, but they must play no part in the actual solution of the mystery.   

  • Secret passages or hidden rooms are all right (if the setting allows it), but do not deserve to be used as an explanation of the murder method.   

  • Avoid unknown Amazonian arrow poisons or newly invented Death-Ray machines, unless as an author you are qualified (scientifically) to justify it (i.e., if Newton had written a mystery based on his laws of Optics, that would be OK, but don’t presume to invent a poison if you don’t even know that aspirin can be fatal.)   

  • Do not use ‘foreigners’ or other aliens as major characters unless you have some real understanding of their culture and mind-set, and they have some relevance to the plot beyond exotic obfuscation.   

  • Avoid accidental solutions, as they are hardly fair in a story of deduction and the presentation of real clues. And please do not inflict on the poor reader one of those mid-book “Mon dieu, how could I not have seen that before” exclamations which sit like undigested food until the end of the mystery.   

  • The criminal should not be someone you have intentionally presented as totally trustworthy. (If he/she is a liar, at least provide some clue to give the reader a chance to spot that.)   

  • All clues must be revealed, although it is perfectly legitimate to disguise them. (But I would draw the line at basing a clue on some misspelling of a word, American vs. British usage, for example, because most books are hardly proofread any more.)   

  • There should but doesn’t have to be a ‘Watson’ or some observing point of view that sees but misinterprets the events under investigation. (Only common sense, otherwise where is the drama?)   

  • Do not try to fool the reader with improbable impersonations, such as a woman posing as a man or vice versa and getting away with it by consummate acting ability, especially when they are deceiving people who know them well. (This doesn’t even work in Shakespeare.) Especially avoid wigs and false whiskers!   

A few more caveats based on this reviewer’s prejudices (another 10 Commandments):


  • Do not try to confuse the reader with elaborate timetables based on train schedules, etc., as there is no guarantee that things like that would ever work out for even the carefullest murderer. (Sod’s or Murphy’s Law.)   

  • Avoid having your Prime Suspect turn out to be the culprit after all, because this is ultimately disappointing (unless you are clever enough to totally reshuffle motives and alibis).   

  • Do not present an ‘impossible crime’ situation without at least attempting to verify its plausibility by experiment. Also try to avoid using an accomplice to abet the criminal’s illusion. (That’s OK for stage magicians with their assistants, but spoils a mystery plot where the villain has to deceive the detective, almost, but without cheating. It makes a lot of sense, too, if you are a villain, not to risk collaboration.)   

  • The murderer should never turn out to be somebody incapable of committing the crime, at least as presented in the lead-up (i.e., invalids in wheelchairs, morons, a person in an intensive-care ward, an astronaut who happened to be in orbit at the time).   

  • A conspiracy involving a hired hit-man, or a mysterious Illuminati cartel, does not belong in a true detective novel. This also includes situations where several suspects are independently up to no good and just happen to be on the scene at the relevant time. (Sod’s Law, again, and a very mechanical manipulation of coincidence for supposedly dramatic purposes — this won’t fool anybody and should be dismissed as mere padding.)   

  • No faking of fingerprints or other forensic details. In spite of their portrayal, even the police a hundred years ago were not as incompetent as they were made out to be. Nowadays, if you want to commit a murder, forget trying such a thing, unless you can afford a good lawyer to screw up the expert witnesses at your trial!   

  • If you are going to talk down to the reader (who is an ignoramus, whereas you are a genius), via your detective, make sure your facts are correct. Twaddle about Egyptology (curse of the pharaoh, etc.) is unacceptable. Informative facts about some obscure subject, however, are beneficial.   

  • Do not present your detective as an ineffectual fool or allow him or her to show any signs of not being superior to the reader or the ‘Watson’ (except to the extent that the detective can have misjudgements and miscalculations for the sake of ‘bonding’ with the reader). An incompetent detective is an actor in a comedy, not a detective story.   

  • Get your details of real police policies and forensic science up to date as far as you can. Unless the book takes place in the classic stranded house-party tradition, there is no way an author can get away with ignoring public procedures, no matter how gifted the detective.   

  • Finally, a personal peeve: Don’t have a large cast of characters and refer to them all by their Christian names, such as Evelyn, Jane, Meg, Charles, and Chris. Who in the hell are you talking about?