I came across interesting if not antiquated literary relic. I don’t necessarily agree with everything on it but it’s a good stab at preparing an agreed framework, not unlike what Asimov’s Three Laws on Robotics did for literature on robots. Here is Fr. Ronald Knox’s famous Commandment list for Detective Novelists (copyright © 1929 Ronald Knox ):

  1. The criminal must be someone mentioned in the early part of the story, but must not be anyone whose thoughts the reader has been allowed to follow.
  2. All supernatural or preternatural agencies are ruled out as a matter of course.
  3. Not more than one secret room or passage is allowable.
  4. No hitherto undiscovered poisons may be used, nor any appliance which will need a long scientific explanation at the end.
  5. No accident must ever help the detective, nor must he ever have an unaccountable intuition which proves to be right.
  6. The detective must not himself commit the crime.
  7. The detective must not light on any clues which are not instantly produced for the inspection of the reader.
  8. The stupid friend of the detective, the Watson, must not conceal any thoughts which pass through his mind; his intelligence must be slightly, but very slightly, below that of the average reader.
  9. Twin brothers, and doubles generally, must not appear unless we have been duly prepared for them.

You will note, of course, that every one of these commandments has been violated at one time or another in a classic mystery novel.

PS: Here is the oath, composed by G. K. Chesterton, of membership in the famous British Detection Club:

“Do you promise that your detectives shall well and truly detect the crimes presented to them using those wits which it may please you to bestow upon them and not placing reliance on nor making use of Divine Revelation, Feminine Intuition, Mumbo Jumbo, Jiggery-Pokery, Coincidence, or Act of God?”