I’ve seen these rules before and found them again here. I prefer mystery novels that follow them, in addition to my own rules(bold):
In 1936, S.S. Van Dine (author of the Philo Vance mysteries) published an article titled «Twenty Rules for Writing Detective Stories.» Obviously, a lot has changed since then — but maybe not as much as we might think. The rules are:
1) The reader should have the same opportunity as the detective to solve the crime.
2) No tricks can be played to mislead the reader unless it is also done to the detective by the criminal.
3) The detective should not have a love interest.
3) A love interest should be a minor part of the story and be necessary for the mystery plot.
4) Neither the detective nor one of the official investigators can turn out to be the criminal.
5) The villain must be found by logical deduction, not luck, accident, or un-motivated confessions.
6) The story must have a detective who also solves the crime (by detection).
7) It must be a murder mystery («the deader the corpse the better»).
8) The solution must come by «naturalistic means»; e.g., no ouija-boards.
9) There can be only one detective; not a team.
9) There must be one main caracter (detective). The number of persons should be as low as possible.
10) The villain has to be someone who plays a prominent part of the story.
11) The culprit can’t be a servant (none of this, «The butler did it.»).
12) There can only be one murderer. The villain could have a helper or «co-plotter,» but only one is going to get the ax in the matter.
13) No secret societies («mafias, et al»). The murderer, too, needs a sporting chance to outwit the detective.
14) The method of the murder must not be beyond plausibility. No super-natural means, nor the introduction of a fictional device or element («super-radium, let us say» is not fair).
15) The truth of the solution must be apparent. The reader should be able to pick the book upon completion and see that the answer was in fact starring at him all the time.
16) The detective «novel» must be just that, no side issues of «literary dallying» or «atmospheric preoccupations.» These devices interfere with the purpose of detective fiction, «which is to state a problem, analyze it» and solve it.
17) The culprit must be an amateur, not a professional criminal.
18) The solution must never be an accident or suicide.
19) Motives for the crime must be personal, not political or professional.
20) All of the following tricks and devices are verboten. They’ve been done to death or are otherwise unfair.
a) Comparing a cigarette butt with the suspect’s cigarette.
b) Using a séance to frighten the culprit into revealing himself.
c) Using phony fingerprints.
d) Using a dummy figure to establish a false alibi.
e) Learning that the culprit was familiar because the dog didn’t bark.
f) Having «the twin» do it.
g) Using knock-out drops.
h) If the murder is in a locked room, it has to be done before the police have actually broken in.
i) Using a word-association test for guilt.
j) Having the solution in a coded message that takes the detective until the end of book to figure out.
My additional rules:
21) The murderer must not be psychotic, but have a reasonable motive.
22) No diaries or the murderer’s thoughts inserted, in italic or otherwise.
23) Neither the victim is allowed to keep a diary.
24) The detective should not be a police.
25) Mystery fiction is should be written either in first person POV or in present tense.
The validation of one or maybe two of these rules may be acceptable, if done well, but if there are more than two broken rules in a novel, I’ll trash it.
Very few mystery novels follow these rules, so I’ve decided that I have to write one myself. It is very hard. That’s why I procrastinate by blogging instead. But I promise you; I will complete it (in Norwegian). And I may want to remove this post when the novel is finished, just so the critics won’t bust me for breaking my own rules 😛