The detective story  is a game. It is more--it is a sporting event. And the author must play  fair with the reader. He can no more resort to trickeries and  deceptions and still retain his honesty than if he cheated in a bridge  game. He must outwit the reader, and hold the reader's interest, through  sheer ingenuity. For the writing of detective stories there are very  definite laws--unwritten, perhaps, but none the less binding: and every  respectable and self-respecting concocter of literary mysteries lives up  to them.

Herewith, then, is a sort of Credo, based  partly on the practice of all the great writers of stories, and partly  on the promptings of the honest author's inner conscience. To wit:

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 

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 some one a bright penny for a  five-dollar gold piece. It's false pretenses.

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 sÈances,  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 gem¸tlich, 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.                                    

a. 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.
b. The bogus spiritualistic sÈance to frighten the culprit into giving himself away.
c. Forged finger-prints.
d. The dummy-figure alibi.

e. The dog that does not bark and thereby reveals the fact that the intruder is familiar.

f. The final pinning of the crime on a twin, or a relative who looks exactly like the suspected, but innocent, person.
g. The hypodermic syringe and the knockout drops.
h. The commission of the murder in a locked room after the police have actually broken in. 
i. The word-association test for guilt.
j. The cipher, or code letter, which is eventually unravelled by the sleuth.

(Found on http://www.thrillingdetective.com/trivia/triv288.html)