Despite his rejection of Holmes and Watson, he agrees with Milne about the value of a Watson-like figure.
In her full-length novels Christie sticks to murder, although several of her short stories feature lesser crimes or no crimes at all. However, Josephine Tey clearly did not feel that a good mystery had to have a corpse and wrote several books, such as Miss Pym Disposes (1946), The Franchise Affair (1948), and To Love and Be Wise (1950), in which either the death was unintentional or no death occurred.
If, as I suspect, Van Dine did model Philo Vance on the early Peter Wimsey, I would have enjoyed seeing his reaction to the penultimate Wimsey novel, Gaudy Night (1935.) In it Sayers broke: Rule 3, a love story; Rule 7, no corpse; Rule 9, two detectives; Rule 11, a servant did it; Rule 16, exploring side-issues.
Sayers proved to be an equal opportunity author in her final Wimsey novel, Busman's Honeymoon (1937), when again a servant committed the crime. And in Five Red Herrings (1931), the death was tried as manslaughter rather than murder. Also she bent Rule 20j in Have His Carcase (1932), when Wimsey and Harriet Vane decode a letter which has an important bearing on the plot.Both Sayers and Christie use the identical twin motif in short stories.
In the 1920s and 1930s spiritualism and seances were popular both as a serious attempt by the bereaved to contact loved ones and as an entertainment. As well as short stories based on spiritualism, Christie uses seances and the ouija board to either set the scene for a murder, as in The Sittaford Mystery (1931) and Dumb Witness (1937); to obscure the truth: The Pale Horse (1961); or to startle a killer into confessing, as in Peril at End House (1932), which definitely breaks Van Dine's Rule 20b. Sayers also uses a phoney séance in Strong Poison (1930), although not to gain a confession.
I do not believe that any of the Golden Age greats deliberately went out of their way to break the rules, they just had fun playing with the permutations of the genre. Christie was particularly inventive, which meant that she broke the rules with exceptional panache. As Anthony Horowitz said of Christie, 'It is as if she was given five or six basic stitches and somehow used them to create the Bayeux Tapestry .' (Introduction to the Hercule Poirot Novels Collection published by the Folio Society.)