The Golden Age of Mystery Fiction

I've been reading and collecting mystery novels most of my life.   I'm going to spend the next year or so re-reading and commenting on my collection.  Most, but not all of them are in the classic tradition of the "Golden Age" mysteries, so I thought a few words on what that tradition is about might be helpful at the start.

 The "Golden Age" of mystery fiction is generally considered to have started in the 1920's, flourished in the period between WWI and WWII and extended into the 1950's, although these dates are not carved in stone by any means.  Golden Age mysteries are usually although not exclusively English, are usually not excessively violent or graphic and are usually puzzles written according to a code of fair play where the reader is given sufficient clues to enable the reader to solve the mystery along with the detective.  

In 1928 Monsignor Ronald A. Knox, a noted mystery author, set down ten commandments for the authors of detective fiction that enumerate the features of this code of fair play.  Those rules are:

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.  Once Agatha Christie had done it, no one else could do it.  

2.  All supernatural or preternatural agencies are ruled out as a matter of course.  If you're writing a ghost story, call it that and not a mystery or detective story.  

3.  Not more than one secret room or passage is allowable.  Father Knox himself violated this one, but points out that he was careful to state that the house had belonged to English Catholics during the period of religious turmoil when a priest's hole or concealed passage could reasonably be expected to be found in such a house.

4.  No hitherto undiscovered poisons may be used, nor any appliance which will need a long scientific explanation in the end.  He gives R. Austin Freeman's detective Dr. Thorndyke a pass on this one, but just barely.

5.  No Chinaman must figure in the story.  He rightly deplores the racist Western assumption of the time that Chinese people were "over-equipped in the matter of brains and under-equipped in the matter of morals."  This rule of course does not apply to Robert van Gulik's Judge Dee mystery novels, which are set in ancient China and in which all of the characters are Chinese.  

6.  No accident must ever help the detective, nor must he ever have an unaccountable intuition which proves to be right.  An intuition or insight based on available evidence that has been disclosed to the reader is acceptable, but not one based on evidence that has been concealed, or on no evidence at all.  

7.  The detective himself must not commit the crime.  This doesn't rule out criminals disguised as or purporting to be detectives, of course.  Agatha busted this one early on, too.  

8.  The detective must not light on any clues which are not instantly produced for the inspection of the reader.  This is the essence of fair play; the reader must have the means necessary to arrive at a logical solution to the puzzle.  

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.  Of course, it's not necessary for the detective to have a Watson at all.  When we get to Conan Doyle, I'll have a few words to say about Dr. Watson, who I believe has been unfairly regarded as stupid.  Less brilliant and less well trained in observation than Holmes, perhaps, but not less intelligent than the average reader.  

10.  Twin brothers, and doubles generally, must not appear unless we have been duly prepared for them.  Really, it's out and out cheating if that happens.  

Source: "A Detective Story Decalogue", Ronald A. Knox, reproduced in The Art of the Mystery Story, A Collection of Critical Essays.  Edited by Howard Haycraft, Grosset & Dunlap, 1946.