In the Next.js App Router, what does prefixing a folder inside app/ with an underscore — for example app/_components/ — do, and how would you serve a URL segment that genuinely starts with an underscore?

## What the underscore does Prefix any folder under `app/` with `_` and Next treats it as **private**: it is excluded from routing, and so is everything nested inside it. The recursion is the part worth stating out loud in an interview — a `page.tsx` deep inside a private folder still produces no URL, because the exclusion applies at the private folder and everything below it is unreachable regardless of what conventions it contains. ``` app/ _components/ Chart.tsx never routable nested/ page.tsx still never routable dashboard/ page.tsx -> /dashboard ``` ## Why bother, when colocation already works? This is the follow-up that separates a memorised answer from an understood one. The App Router routes only on a fixed set of reserved filenames, so an ordinary module dropped into a route folder is already invisible to routing. You do not *need* a private folder to keep a component next to its page. What the underscore adds is a guarantee scoped to a directory rather than to individual files: - **Intent.** The name says "implementation, not a route" to every reader and reviewer, without them having to know the reserved-filename list. - **Protection against future mistakes.** Colocation is safe only while nobody adds a reserved filename. Someone creating `page.tsx` inside a shared folder to sketch something out publishes a route by accident. Inside a private folder they cannot. - **Insulation from the convention namespace.** Reserved filenames are Next's namespace and it has grown over time. A private folder is a subtree the routing layer never inspects. - **Grouping unrelated to a single route.** Colocation naturally holds code belonging to *this* segment. A private folder is a clean home for utilities shared by several segments while still living under `app/`. The common patterns are a private folder near the root for cross-cutting internals — `app/_lib/`, `app/_components/` — and private folders inside a feature area for code shared by its child segments. ## Serving a literal leading underscore Occasionally a URL genuinely needs to begin with an underscore. Since a raw `_` prefix means "private", the escape hatch is the URL-encoded form: name the folder `%5F` followed by the rest of the segment. ``` app/ %5Fdrafts/ page.tsx -> /_drafts ``` `%5F` is the percent-encoding of `_`, and Next decodes it when building the route, producing a public segment whose URL starts with an underscore. This is niche, but it is exactly the kind of detail an interviewer uses to check whether you have read the conventions or only pattern-matched them. ## How this differs from the other folder-name convention Do not confuse a private folder with a folder whose name is wrapped in parentheses. Both are folder-name conventions and both change routing, but in opposite directions: - **Private folder, `_name`:** the subtree leaves routing altogether. Nothing under it is reachable. - **Parenthesised folder, `(name)`:** the subtree stays fully routable; the folder simply contributes no segment to the URL, so it organises files without changing paths. One removes routes, the other removes a path segment. Mixing them up in an interview reads as unfamiliarity with the directory conventions generally. ## Practical guidance Do not sprinkle private folders everywhere. A route's own component sitting beside its `page.tsx` is idiomatic and already safe; wrapping every helper in `_` adds a directory level for nothing. Reach for the underscore when a directory is shared, when it is large enough that a stray reserved filename is a realistic risk, or when you want the name itself to tell a newcomer that nothing inside is a route. And remember the recursion — moving a genuine route folder underneath a private one silently removes it from the application, which is a confusing bug to chase precisely because the code still compiles and the files look right.