The higher bar assumes a sceptical reader, one who must be persuaded that the detour was needed and who holds the separation against the book until it is justified. That reader exists, but she is not the only reader the form has had. An Elizabethan audience expected a prologue and would have marked its absence; a reader of the eighteenth-century novel expected an editor to say where the papers came from, because a book asserting invented events with no account of how they were known was not yet a settled convention; a reader who chooses a long fantasy has, by choosing it, accepted an overture that a general reader might refuse. Change the reader and the bar moves. The standard is real but it is not natural, and the distinction matters because the two uses of the bar lead to different revisions.
As an instrument, the bar makes a writer state a choice that would otherwise be made by default: why this material, here, outside. Used that way it rarely removes a good prologue and often improves one, because the statement either holds or shows where the material belongs. Mistaken for a fact about narrative, it becomes the market’s suspicion in the language of craft, and it licenses the removal of prologues such as those before Romeo and Juliet, Frankenstein and Invisible Man, none of which would survive a rule against the form.
The two pressures often point the same way, since a prologue that cannot say what its separation buys is usually both a weak prologue and a submission liability. When they diverge, the writer should know which one she is answering. A quiet prologue that serves the book and costs a sale is a cost she can choose to pay or to avoid by absorption; a loud one that wins the sale and misreads the book is a cost her readers pay for her. The tests in this chapter cannot decide between those, but they can make the decision visible as hers.