Rules for Mystery Writers

People often talk about how romances are formulaic, but it looks like they might have nothing on mystery stories. many of you know I like Ilona Andrews's books and blog, she drew my attention to Father Knox and his rules.

In 1929 Monsignor Ronald A. Knox a British clergyman, editor, a literary critic, a humourist and a detective story writer complied a list of what makes a good mystery. We'll never know quite how serious he was about the list but...

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 supernaural 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.
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.

NB: For the record I don't think genre fiction is so easy any old sod can write it. I write romance and it's anything but easy. One of the things that's hardest is readers know how the story is going to end (romance here, other genres have their own quirks), yet as the writer you still have to make your story interesting enough to take the reader from the first to the last page. My view is in some respects genre fiction is harder to write. So, yes, I've had a giggle at the rules above, but it's a giggle kindly meant.