1 The reader must have equal opportunity with the detective for solving the mystery.
2 No willful tricks or deceptions may be placed on the reader other than those played by the criminal on the detective.
3 There must be no love interest.
4 The detective himself, or one of the official investigators, should never turn out to be the culprit.
5 The culprit must be determined by logical deduction – not by accident, coincidence or unmotivated confession.
6 The detective novel must have a detective in it.
7 There simply must be a corpse in a detective novel.
8 The problem of the crime must be solved by strictly naturalistic means.
9 There must be but one detective.
10 The culprit must turn out to be a person who has played a more or less prominent part in the story.
11 A servant must not be chosen as the culprit.
12 There must be but one culprit no matter how many murders are committed.
13 Secret societies have no place in a detective story.
14 The method of murder, and the means of detecting it, must be rational and scientific.
15 The truth of the problem must be at all times apparent provided the reader is shrewd enough to see it.
16 A detective novel should contain no long descriptive passages, no literary dallying with side issues, no subtly worked-out character analyses, and no ‘atmospheric’ preoccupations.
17 A professional criminal must never be shouldered with the guilt in a detective novel.
18 A crime in a detective story must never turn out to be an accident or a suicide.
19 The motives for all the crimes in detective stories should be personal.
20 A list of devices, which no self-respecting detective story writer should avail himself of including, among others:
Ronald Knox’s Detective Story Decalogue
Monsignor Ronald Knox (1888–1957) was a priest and classical scholar who wrote six detective novels between 1925 and 1937. He created the insurance investigator detective Miles Bredon, and considered the detective story such a serious game between writer and reader that in some of his novels he provided page references to his clues. When he edited a collection of short stories, The Best Detective Stories of 1928, his Introduction included a ‘Detective Story Decalogue’. These distilled the essence of a detective story, as distinct from the thriller, into ten cogent sentences:
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.
2 All supernatural 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 long scientific explanation at the end.
5 No Chinamen must figure in the story.
6 No accident must ever help the detective, nor must he ever have an unaccountable intuition that proves to be right.
7 The detective must not himself commit the crime.
8 The detective must not light on any clues that are not instantly disclosed to the reader.
9 The stupid friend of the detective, the Watson, must not conceal any thoughts that 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.
But as will be seen from a survey of Christie’s output, many of the Rules laid down by both Knox and Van Dine were ingeniously ignored and often gleefully broken by the Queen of Crime. Her infringement was, in most cases, instinctive rather than premeditated; and her skill was such that she managed to do so while still remaining faithful to the basic tenets of detective fiction.
Agatha Christie’s Rule of Three
In order to examine these Rules, and Christie’s approach to them, I have grouped together Rules common to both lists and have divided them into categories:
Fairness
Both lists are very concerned with Fairness to the reader in the provision of information necessary to the solution, and with good reason; this is the essence of detective fiction and the element that distinguishes it from other branches of crime writing. Van Dine 1 and Knox 8 are, essentially, the same rule while Van Dine 2, 5, 15 and Knox 9 elaborate this concept.
Van Dine 1. The reader must have equal opportunity with the detective for solving the mystery.
Knox 8. The detective must not light on any clues that are not instantly disclosed to the reader.
Christie did not break these essentially identical rules, mainly because she did not need to. She was quite happy to provide the clue, firm in the knowledge that, in the words of her great contemporary R. Austin Freeman, ‘the reader would mislead himself’. After all, how many readers will properly interpret the clue of the torn letter in Lord Edgware Dies, or the bottle of nail polish in Death on the Nile, or the ‘shepherd, not the shepherdess’ in A Murder is Announced? Or who will correctly appreciate the significance of the smashed bottle in Evil under the Sun, or the initialled handkerchief in Murder on the Orient Express, or the smell of turpentine in After the Funeral?
Knox 9. The stupid friend of the detective, the Watson, must not conceal any thoughts that pass through his mind; his intelligence must be slightly, but very slightly, below that of the average reader.
Hastings has been dubbed ‘the stupidest of Watsons’ and there are times when we wonder how Poirot endured his intellectual company. And, of course, Agatha Christie herself tired of him and banished him to Argentina in 1937 after Dumb Witness, although he was to return for Curtain: Poirot’s Last Case, written during the Second World War but not published