Certain conventions and clichés were established that limited any surprises on the part of the reader to the details of the plot and, primarily, to the identity of the murderer. The majority of novels of that era were "whodunits", and several authors excelled, after misleading their readers successfully, in revealing the least likely suspect convincingly as the villain. There was also a predilection for certain casts of characters and certain settings in a secluded English country house and its upper-class inhabitants. The rules of the game - and Golden Age mysteries were considered games - were codified in 1929 by Ronald Knox. According to Knox, a detective story Knox's "Ten Commandments" are as follows:
The criminal must be mentioned in the early part of the story, but must not be anyone whose thoughts the reader has been allowed to know.
No hitherto undiscovered poisons may be used, nor any appliance which will need a long scientific explanation at the end.
No Chinaman must figure in the story.
No accident must ever help the detective, nor must he ever have an unaccountable intuition which proves to be right.
The detective himself must not commit the crime.
The detective is bound to declare any clues which he may discover.
The "sidekick" of the detective, the Watson, must not conceal from the reader any thoughts which pass through his mind: his intelligence must be slightly, but very slightly, below that of the average reader.
Twin brothers, and doubles generally, must not appear unless we have been duly prepared for them.
A similar but more detailed list of prerequisites was prepared by S. S. Van Dine in an article entitled "Twenty Rules for Writing Detective Stories" which appeared in The American Magazine in September 1928. They are commonly referred to as Van Dine's Commandments.
Decline and fall
The outbreak of the Second World War is often taken as a beginning of the end for the light-hearted, straightforward "whodunit" of the Golden Age. But as Ian Ousby writes, the Golden Age Attacks on the genre were made by the influential writer and critic Julian Symons, Edmund Wilson, and Raymond Chandler. But in sheer number of sales — particularly those of Agatha Christie — modern detective fiction has never approached the popularity of Golden Age writing.
Enduring influence
Current writing influenced by the Golden Age style is often referred to as "cosy" mystery writing, as distinct from the "hardboiled" style popular in the United States. Recent writers working in this style include Sarah Caudwell, Ruth Dudley Edwards, Peter Lovesey and Simon Brett. Television series that emulate the style include Murder, She Wrote and Midsomer Murders. Films and TV series based on the classic Golden Age novels continue to be produced. The Country house mystery was a popular genre of English detective fiction in the 1920s and 1930s; set in the residences of the gentry and often involving a murder in a country house temporarily isolated by a snowstorm or similar with the suspects all at a weekend house party. The board gameCluedo relies on the structure of the country house mystery. From the late 1980s to the early 1990s, not a few mystery writers who were influenced by the Golden Age style made their debut one after another in Japan. They are referred to as "new traditionalists" or "new orthodox school". Representative "new traditionalists" include writers such as Yukito Ayatsuji, Gosho Aoyama, Rintaro Norizuki and Taku Ashibe.