What is grammatically correct narrower or more narrow?

Both "narrower" and "more narrow" are grammatically correct, but "narrower" is generally preferred.

Here's why:

* "Narrower" is the comparative form of the adjective "narrow." It's the simpler and more concise way to express the comparison.

* "More narrow" is grammatically correct but less common. It uses the "more" + adjective structure, which is more common with adjectives that don't have a regular comparative form (e.g., more beautiful, more difficult).

In most cases, "narrower" is the better choice.

However, there might be situations where "more narrow" sounds more natural or emphasizes the degree of narrowness. For example:

* "This road is more narrow than I expected." (Emphasizing the unexpected level of narrowness)

Ultimately, the best choice depends on the context and desired emphasis.