There must at the dénouement be surprise, & yet the surprise must not be complete. Hence the criminal must not be an outsider of whom we've hardly heard. He must not be || Nor the detective, unless the detective was made out the sort of man, one might have suspected; the criminal must not be || . Nor the nice young man, or nice young woman to whom during the whole course of the story we wish happiness, on || : On the other hand he must not be the
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one obvious suspect. These rules are very stringent & leave only a very small || narrow margin for the writer, as it were a tiny dance floor on which to perform.