Way back when (well... about the 1920s), detective-crime fiction was a rather popular genre - most notably in printed works. The genre that people such as Agatha Christie pioneered had, at this point, become something of a game - much as it is today, although it could often now be described more as a puzzle than a game. Nonetheless, puzzles and games alike require rules. Ronald Knox was one of Agatha Christie's contemporaries, and suggested the following list of ten rules of murder mysteries:
- The criminal must be mentioned in the early part of the story, and must not be anyone whose thoughts the reader has been allowed to know. In other words, no surprises - Chekhov's Gun must, after all, be put in place before it is fired.
- All supernatural or preternatural agencies are ruled out as a matter of course. If it defies (current) human logic, it's out. Of course, one must consider the time in which the fiction was written - a book from the 2000s that involves cyber-crime in some way could be considered supernatural for the 1920s. But ESP is always a no-go.
- Not more than one secret room or passage is allowable. Too many, and you're starting to stray into "defying human logic" territory. In-universe, why have that many secret passageways in the first place? Out of universe, it's far too easy to write an undiscovered secret passageway in as a plausible explanation at the end of the book, it gets hackneyed fast. But it also makes it impossible to guess based on human logic.
- No hitherto undiscovered poisons may be used, nor any appliance which will need a long scientific explanation at the end. Arsenic? Valid. Hydrofluoric acid? (Disturbing, but) valid. A chemical agent that attacks the lymph nodes, eyesight, and left big toe of its victims? Whatever its name is, it's not to be used.
No Chinaman must figure in the story. This rule no longer makes sense. Let's move on.- No accident must ever help the detective, nor must he ever have an unaccountable intuition which proves to be right. I keep returning to the word, but "logic" is the reason again. It might work for Johnny English but it shouldn't work for puzzles.
- The detective himself must not commit the crime. After all, the detective is the good guy, not the bad guy.
- The detective is bound to declare any clues he may discover. Otherwise, it's simply unfair on the reader - especially if it's the clue that the entire mystery relies on.
- The "sidekick" of the detective must not conceal any thoughts which pass through their mind; their intelligence must be very slightly below that of the average reader. I do actually appreciate this rule when it's broken - the second half, at least. But again, concealment of thoughts is unfair - the sidekick often brings up ideas that can be, or indeed are, quashed by the detective and their logic - a useful tool for the author.
- Twin brothers, and doubles generally, must not appear unless we have been duly prepared for them. Which twin did it? (Aside from, obviously, the evil twin.) It's all too easy to pull a fast one on the audience if we allow any kind of body double or lookalike.
Though these days detective fiction per se is a little more niche, it does still exist in many forms, notably TV adaptations of Christie's work, as well as those from other authors; the Brits tend to be very good at giving the world detective fiction-esque TV shows (viz. Midsomer Murders, Rosemary and Thyme, and oh my gods Broadchurch). This is, of course, not to mention works in other media - there are some excellent video games, especially visual novel type games, that espouse all of the above rules. In saying that, my preference is for those works that subvert or outright break one or more of these rules (and one of my favourite video games of late is notable for breaking nine out of the ten - #5 is considerably altered then also broken) as it breaks the monotony and predictability of these tales. That's not to say that they're not fair in their own right, though - just with a couple of tweaks.