S.S.Van Dine's 20 Rules for Detective Fiction

1. The reader must have equal opportunity with the detective for solving the mystery. All clues must be plainly stated and described.
2. No wilful tricks or deceptions may be played on the reader other than those played legitimately by the criminal on the detective himself.
3. There must be no love interest in the story. To introduce amour is to clutter up a purely intellectual experience with irrelevant sentiment. The business in hand is to bring a criminal to the bar of justice, not to bring a lovelorn couple to the hymeneal altar.
4. The detective himself, or one of the official investigators, should never turn out to be the culprit. This is bald trickery, on a par with offering someone a bright penny for a five-dollar gold piece. It's false pretences.
5. The culprit must be determined by logical deductions--not by accident or coincidence or unmotivated confession. To solve a criminal problem in this latter fashion is like sending the reader on a deliberate wild-goose chase, and then telling him, after he has failed, that you had the object of his search up your sleeve all the time. Such an author is no better than a practical joker.
6. The detective novel must have a detective in it; and a detective is not a detective unless he detects. His function is to gather clues that will eventually lead to the person who did the dirty work in the first chapter; and if the detective does not reach his conclusions through an analysis of those clues, he has no more solved his problem than the schoolboy who gets his answer out of the back of the arithmetic.
7. There simply must be a corpse in a detective novel, and the deader the corpse the better. No lesser crime than murder will suffice. Three hundred pages is far too much pother for a crime other than murder. After all, the reader's trouble and expenditure of energy must be rewarded. Americans are essentially humane, and therefore a tiptop murder arouses their sense of vengeance and horror. They wish to bring the perpetrator to justice; and when "murder most foul, as in the best it is," has been committed, the chase is on with all the righteous enthusiasm of which the thrice gentle reader is capable.
8. The problem of the crime must be solved by strictly naturalistic means. Such methods for learning the truth as slate-writing, ouija-boards, mind-reading, spiritualistic séances, crystal-gazing, and the like, are taboo. A reader has a chance when matching his wits with a rationalistic detective, but if he must compete with the world of spirits and go chasing about the fourth dimension of metaphysics, he is defeated ab initio.
9. There must be but one detective – that is, but one protagonist of deduction – one deus ex machine. To bring the minds of three or four, or sometimes a gang of detectives to bear on a problem is not only to disperse the interest and break the direct thread of logic, but to take an unfair advantage of the reader, who, at the outset, pits his mind against that of the detective and proceeds to do mental battle. If there is more than one detective the reader doesn't know who his co-deductor is. It's like making the reader run a race with a relay team.
10. The culprit must turn out to be a person who has played a more or less prominent part in the story – that is, a person with whom the reader is familiar and in whom he takes an interest. For a writer to fasten the crime, in the final chapter, on a stranger or person who has played a wholly unimportant part in the tale, is to confess to his inability to match wits with the reader.
11. Servants – such as butlers, footmen, valets, game-keepers, cooks, and the like – must not be chosen by the author as the culprit. This is begging a noble question. It is a too easy solution. It is unsatisfactory, and makes the reader feel that his time has been wasted. The culprit must be a decidedly worth-while person--one that wouldn't ordinarily come under suspicion; for if the crime was the sordid work of a menial, the author would have had no business to embalm it in book-form.
12. There must be but one culprit, no matter how many murders are committed. The culprit may, of course, have a minor helper or co-plotter; but the entire onus must rest on one pair of shoulders: the entire indignation of the reader must be permitted to concentrate on a single black nature.
13. Secret societies, camorras, mafias, et al., have no place in a detective story. Here the author gets into adventure fiction and secret-service romance. A fascinating and truly beautiful murder is irremediably spoiled by any such wholesale culpability. To be sure, the murderer in a detective novel should be given a sporting chance, but it is going too far to grant him a secret society (with its ubiquitous havens, mass protection, etc.) to fall back on. No high-class, self-respecting murderer would want such odds in his jousting-bout with the police.
14. The method of murder, and the means of detecting it, must be rational and scientific. That is to say, pseudo-science and purely imaginative and speculative devices are not to be tolerated in the roman policier. For instance, the murder of a victim by a newly found element--a super-radium, let us say--is not a legitimate problem. Nor may a rare and unknown drug, which has its existence only in the author's imagination, be administered. A detective-story writer must limit himself, toxicologically speaking, to the pharmacopoeia. Once an author soars into the realm of fantasy, in the Jules Verne manner, he is outside the bounds of detective fiction, cavorting in the uncharted reaches of adventure.
15. The truth of the problem must at all times be apparent – provided the reader is shrewd enough to see it. By this I mean that if the reader, after learning the explanation for the crime, should reread the book, he would see that the solution had, in a sense, been staring him in the face--that all the clues really pointed to the culprit--and that, if he had been as clever as the detective, he could have solved the mystery himself without going on to the final chapter. That the clever reader does often thus solve the problem goes without saying. And one of my basic theories of detective fiction is that, if a detective story is fairly and legitimately constructed, it is impossible to keep the solution from all readers. There will inevitably be a certain number of them just as shrewd as the author; and if the author has shown the proper sportsmanship and honesty in his statement and projection of the crime and its clues, these perspicacious readers will be able, by analysis, elimination and logic, to put their finger on the culprit as soon as the detective does. And herein lies the zest of the game. Herein we have an explanation for the fact that readers who would spurn the ordinary "popular" novel will read detective stories unblushingly.
16. A detective novel should contain no long descriptive passages, no literary dallying with side-issues, no subtly worked-out character analyses, no "atmospheric" preoccupations. Such matters have no vital place in a record of crime and deduction. They hold up the action, and introduce issues irrelevant to the main purpose, which is to state a problem, analyze it, and bring it to a successful conclusion. To be sure, there must be a sufficient descriptiveness and character delineation to give the novel verisimilitude; but when an author of a detective story has reached that literary point where he has created a gripping sense of reality and enlisted the reader's interest and sympathy in the characters and the problem, he has gone as far in the purely "literary" technique as is legitimate and compatible with the needs of a criminal-problem document. A detective story is a grim business, and the reader goes to it, not for literary furbelows and style and beautiful descriptions and the projection of moods, but for mental stimulation and intellectual activity--just as he goes to a ball game or to a cross-word puzzle. Lectures between innings at the Polo Grounds on the beauties of nature would scarcely enhance the interest in the struggle between two contesting baseball nines; and dissertations on etymology and orthography interspersed in the definitions of a cross-word puzzle would tend only to irritate the solver bent on making the words interlock correctly.
17. A professional criminal must never be shouldered with the guilt of a crime in a detective story. Crimes by house-breakers and bandits are the province of the police department--not of authors and brilliant amateur detectives. Such crimes belong to the routine work of the Homicide Bureaus. A really fascinating crime is one committed by a pillar of a church, or a spinster noted for her charities.
18. A crime in a detective story must never turn out to be an accident or a suicide. To end an odyssey of sleuthing with such an anti-climax is to play an unpardonable trick on the reader. If a book-buyer should demand his two dollars back on the ground that the crime was a fake, any court with a sense of justice would decide in his favor and add a stinging reprimand to the author who thus hoodwinked a trusting and kind-hearted reader.
19. The motives for all crimes in detective stories should be personal. International plottings and war politics belong in a different category of fiction--in secret-service tales, for instance. But a murder story must be kept gemütlich, so to speak. It must reflect the reader's everyday experiences, and give him a certain outlet for his own repressed desires and emotions.
20. And (to give my Credo an even score of items) I herewith list a few of the devices which no self-respecting detective-story writer will now avail himself of. They have been employed too often, and are familiar to all true lovers of literary crime. To use them is a confession of the author's ineptitude and lack of originality:
• Determining the identity of the culprit by comparing the butt of a cigarette left at the scene of the crime with the brand smoked by a suspect.
• The bogus spiritualistic séance to frighten the culprit into giving himself away.
• Forged finger-prints.
• The dummy-figure alibi.
• The dog that does not bark and thereby reveals the fact that the intruder is familiar.
• The final pinning of the crime on a twin, or a relative who looks exactly like the suspected, but innocent, person.
• The hypodermic syringe and the knockout drops.
• The commission of the murder in a locked room after the police have actually broken in.
• The word-association test for guilt.
• The cipher, or code letter, which is eventually unravelled by the sleuth.