Era of classic murder mystery novels in the 1920/30s that follow Knox's "Ten Commandments:"
1) 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.
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.
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 (a reaction to, and criticism of, racial cliches prevalent in 1920s English writing).
6) No accident must ever help the detective, nor must he ever have an unaccountable intuition which proves to be right.
7) The detective himself must not commit the crime.
8) The detective is bound to declare any clues which he may discover.
9) 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.
10)Twin brothers, and doubles generally, must not appear unless we have been duly prepared for them.
Protagonist is an antihero detective who has been rendered cynical by the violence of organised crime that flourished during Prohibition (1920-1933) and its aftermath. Focus is on action and gritty realism with a tough/unsentimental style and graphic sex/violence.
Crime fiction where right and wrong are not clearly defined and the protagonist is seriously and often tragically flawed. Usually the protagonist is tied directly to the crime in some way, not an outsider called to solve the mystery.