I'm currently reading Kathryn Harkup's 'A Is For Arsenic: The Poisons of Agatha Christie'. In it she mentions that in 1928, Knox (1888-1957) wrote a set of ten rules that crime writers were expected to adhere to in spirit of fairness to the reader.

Knox's Detective Story Decalogue:

1. The criminal must be mentioned in the early part of the story, but must not be anyone whose thoughts the reader has been allowed to know.
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 Chinaman must figure in the story. (Note: This is a reference to the common use of heavily stereotyped Asian characters in detective fiction of the time.)
6. No accident must ever help the detective, nor must he ever have an unaccountable intuition which proves to be right.
7. The detective himself must not commit the crime.
8. The detective is bound to declare any clues which he may discover.
9. The "sidekick" of the detective, the Watson, must not conceal from the reader any thoughts which pass through his mind: his intelligence must be slightly, but very slightly, below that of the average reader.
10. Twin brothers, and doubles generally, must not appear unless we have been duly prepared for them

I copied these from the Wiki entry to save typing but Harkup's wording is more or less the same.

I'd love to have a chat among this forum about these rules. Are there books within the quest that adhere to these? Ones that subvert these rules? Are there other books by authors from the quest that resonate with these roles? Which rules do we see most flouted? Which feel most significant for keeping things fair to the reader? What do you think of the list generally? Are there books from our wider reading that these rules bring to mind and could they be additions to the quest or books for future informal read-alongs? Really... Any comments/thoughts/ramblings welcome! I don't think I'd heard of them before so I was really interested to bring them to the group, so to speak, and get other people's thoughts!