S. S. Van Dine was a pseudonym used by the American mystery writer Willard Huntington Wright. In his essay “Twenty Rules for Writing Detective Stories,” Van Dine outlined what he considered to be the “rules of fair play” for mystery writers. These rules were intended to guide writers in creating stories that were fair to the reader and allowed them to solve the mystery along with the detective.

Here are the 20 rules as outlined by Van Dine:

1. The reader must have equal opportunity with the detective for solving the mystery. All clues must be plainly stated and described.
2. No willful tricks or deceptions may be placed 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. The only romance allowed is that incidental to the plot.
4. The detective himself must not commit the crime.
5. The detective is bound to declare any clues which he may discover.
6. The “sidekick” of the detective, the Watson, must not conceal from the reader any thoughts which pass through his mind.
7. The detective must not himself solve the mystery by any accident or coincidence not before the reader has had the opportunity to solve it himself.
8. The detective must not withhold any vital information from the reader.
9. The solution must be logical and not be a mere coincidence or accident.
10. The detective must not solve the mystery by means of intuition, imagination, or “unscientific” methods.
11. There must be only one detective, either official or private.
12. The culprit must be determined by logical deductions, not by accident or coincidence.
13. The culprit must be a character whose actions are consistent with his or her personality and motives.
14. The amateur detective must not be used.
15. The detective novel must have a detective, either amateur or professional.
16. The crime must be murder.
17. The method of the murder must be reasonable, and the scene must be a fitting one.
18. There must be no secret passages or hidden rooms.
19. The detective himself must not turn out to be the culprit.
20. The ending must be happy and just.

Van Dine’s rules have been both praised and criticized over the years, and many modern writers have ignored or broken some of these rules in their own works. However, these rules remain an interesting historical artifact of the Golden Age of detective fiction and continue to be studied and discussed by mystery writers and readers alike.