The Fair Play Mystery or Fair Play Whodunnit is a common form of mystery writing wherein the writer gives the reader all of the clues necessary to solve the mystery before or at the same time as the fictional detective. The rules for the fair play mystery were first codified in 1928, when mystery writer Father Ronald Knox created a "Ten Commandments" of plot devices commonly referred to as Knox's Decalogue. These "rules" were commonly used and abused by members of the British Detective Club, and are still in play today. They are as follows:
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, and such a passage may only be in a house or building for which it is appropriate by age or purpose.
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.
*Though the term "Chinaman" is now obsolete and generally considered offensive, its inclusion here was most likely not a case of racism, but rather a reaction against the cliched Yellow Peril villains that were ubiquitous in the crime fiction of the time. The worst of modern American crime fiction is riddled with the same cliched characters.