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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 follow.
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All supernatural or preternatural agencies are ruled out as a matter of course.
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Not more than one secret room or passage is allowable.
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No hitherto undiscovered poisons may be used, nor any appliance which will need a long scientific explanation at the end.
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No Chinaman must figure in the story.
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No accident must ever help the detective, nor must he ever have an unaccountable intuition which proves to be right.
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The detective himself must not commit the crime.
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The detective is bound to declare any clues upon which he may happen to light.
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The stupid friend 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.
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Twin brothers, and doubles generally, must not appear unless we have been duly prepared for them.