The borrowing of features between dialects is the basis of the wave model of language change.
Loanwords must therefore be carefully distinguished from cognates—i.e., similarities between languages that are the result of shared inheritance from a common ancestor.
Unlike cognates, borrowing may take place between languages that are unrelated to each other and have no common origin.
Historical linguists occasionally appeal to borrowing to explain apparent exceptions to the regularity of sound change.
However, some apparent exceptions exist: for instance, the earlier phoneme /f/ at the beginning of a word appears to have become /v/ in English vat, vane, and vixen (from Old English fatu, fana, and fyxin respectively), but not in other words beginning with /f/.