Which is correct?
The United States _______ Research Laboratory (NRL) is the corporate research laboratory for the United States Navy and the United States Marine Corps.
Naval refers to something related to the navy. Navy Knowledge Online (NKO), for example, is a naval education website.
A navel, on the other hand, is a bellybutton. So a navel research lab would study matters related to bellybuttons. (Navel is also a type of orange, so the lab might study those instead. But that wouldn’t be as funny.)
Navel vs. naval is covered in Paul Brians’ Common Errors in English Usage.
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