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An outline is a roadmap. It helps you decide the overall shape of the novel. It does not lock you into that structure if you stumble upon something interesting. Thinking of it as a roadmap is a useful analogy. When you are on a road trip, you discover things along the way, even if you know exactly where you are going. Sometimes, you even get off the road, either because you see something that interests you more, or because there’s a roadblock. Having an outline can remind you of where you wanted to go with the novel so that when those detours come up, you can evaluate if they are worth it.