The way I understand hook, it's a device, it's a literary technique, it's both. I think it's widely misunderstood though. For many writers a hook is an action opening, a device that might not hook from being overblown. A hook in a chapter ending transitions to the next chapter, but not necessarily as a cliffhanger, which is a widely deprecated device. An opening hook could also be a thematic opening. The opening sentence of Jane Austen's Pride and Prejudice is a thematic opening hook.
"It is a truth universally acknowledged, that a single man in possession of a good fortune, must be in want of a wife."
An opening hook ought to introduce a central suspense question based upon a main dramatic complication, or in some circumstances a bridging complication related to the main complication, that, regardless, artfully poses some iteration of the basic suspense question in readers' minds. What will happen next? Artfully posing the question and artfully delaying the answer is the height of narrative arts. Pride and Prejudice's opening sentence does the job admirably. Is the single man in possession of a good fortune really in want of a wife? Oh my, what will happen next? Complications ensue.
Chapters artfully pose and answer minor suspense questions and leave the answer to the central suspense question in doubt. Ending a chapter with the central suspense question unanswered, except the final chapter, compels readers on to the next chapter by itself.
Within a chapter in order to move a plot forward a protagonist must answer a pending minor suspense question that might pose another or not. Tension can only build so high without becoming unsustainable. Occassional relief is necessary so that outcome remains in doubt, so that hope of a favorable outcome is not outweighed by certainty of failure. A protagonist wandering about exploring the countryside prospecting for answers must find minor answers that build up to answering the central question a dramatic complication poses; otherwise, it's a travelogue, an anecdote, a vignette without plot progress.
I think I could go up to three or so chapters with a protagonist not answering a minor suspense question satisfactorily, but no is itself an answer. Will they or won't they is amatory romance's central suspense question. Three refusals is a good number. "What I tell you three times is true." Aristotle Poetics. The value of three rising action scene chapters revealed and after a major turn, a reversal, in a tragic crisis, three falling action scene chapters. Less than three is open to question efforts were truly exhausted. More than three becomes episodic. In three major scenes, the protagonist has given all the best efforts to win a love interest and failed. The tragic crisis is the discovery, realization all is lost, which is the major middle turn. Though there should be a clue, perhaps a foreshadowed hint that a slim hope remains. The major opening turn is perhaps a stranger comes to town or a native goes to a strange town, the love interest is discovered.
The amatory romance convention is they will hook up in the ending; otherwise, readers feel disappointment rather than satisfaction. All is lost in the middle. Wait. There's a renewed spark of hope. The love interest is following a logical decision, but the heart will not be denied. Getting there in falling action scenes artfully delays the protagonist winning over the love interest. The protagonist discovers the love interest's true sentiment. All is known. Then it's only a matter of eroding resistance. Formulaic, I know. That's the convention of convention-driven genres.
Other genres' artfully posed suspense questions;
Mystery, who done it.
Psychological or Spy Thriller, will the mundane terror be stopped. (Mundane meaning earthly as opposed to metaphysical or supernatural, not boring.)
Western, will the rogue hero put an end to the villain.
Horror, will the supernatural or metaphysical terror be stopped.
Fantasy, will the hero or heroine achieve the goal.
Science fiction, incorporates a gamut of the above based on fantastical technological, scientific, and/or social situations.
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