1. The detective must be memorable
He/she must be clever and a bit out of the ordinary. The detective must have some small habit, mannerisms, eccentricity, interest, or talent that sets them apart from the crowd.
2. The crime must be significant
It must be constructed around a murder or a great theft.
3. The criminal must be a worthy opponent
If fictional detectives are to show off their considerable skills, they must match wits with adversaries of equal cleverness. The mind of the criminal is often the intellectual equal of the detective's. The conflict becomes a battle of intellects between the detective, opponent, and the reader.
4. All the suspects, including the criminal, must be presented early in the story
The reader must be able to safely assume that the perpetrator of the crime is one of the main characters in the story, not someone whom the author is going to slip in on the unsuspecting reader in the next-to-last chapter.
5. All clues discovered by the detective must be made available to the reader
This is called a "fair play" rule. The reader must be given the same opportunity to solve the crime as the detective. An author may deliberately mislead the reader, using a red herring, as long as his fictional detective is similarly deceived.
6. The solution must appear logical and obvious when the detective explains how the crime was solved.
The reader must see how all the information fits together.
