Knox's 10 Rules of Detective Fiction

In 1930 a small group of mainly English and Irish detective fiction writers (including Agatha Christie, Dorothy L.Sayers, E.C.Bentley, Emma Orczy, and G.K. Chesterton formed the Detection Club, a kind of dinner club and guild, based in London, England. The club remains an invitation only organisation, and one of the main caveats is that members adhere to Knox's 10 rules of detective fiction - though the rules are less strict these days, but I think they are of great help to new cozy mystery writers like myself. I hope you will find them useful too.  

Detection Club Oath

"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?[1]"

Knox's 10 Commandments of Detective Fiction Writing

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 Chinaman must figure in the story. (I told they were dated!)
6. No accident must ever help the detective, nor must he ever have an unaccountable intuition which proves to be right.
7. The detective must not himself commit the crime.
8. The detective must not light on any clues which are not instantly produced for the inspection of the reader.
9. 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.
10. Twin brothers, and doubles generally, must not appear unless we have been duly prepared for them.