In the 1920s, Ronald Knox, Sherlock Holmes enthusiast and himself author of several detective stories, was engaged in trying to codify the ground rules of detective stories. Like others of his generation, he saw the genre as a kind of game – ‘a mystery whose elements are clearly presented to the reader at an early stage in the proceedings, and whose nature is such as to arouse curiosity, a curiosity which is gratified at the end.’
The ‘Ten Commandments of Detection’, laid down by Ronald Knox in 1928 are in this link ...
http://www.crimeculture.com/359/Rules.htm