Why do French and some other languages have nasalized vowels?

Nasalized vowels occur when the airstream is expelled partly through the nose and partly through the mouth, a process resulting from the lowering of the soft palate (velum) at the back of the mouth.1

In many Romance languages, a following nasal consonant typically exerts a raising or closing influence on a preceding vowel. However, in French and Portuguese, this interaction evolved into phonological nasalization, where nasal vowels became distinct phonemes—meaning they serve to differentiate the meanings of words, such as the French pin ("pine") versus paix ("peace").2

While the exact origins are debated, this development was likely noticeable by the 10th century. Some linguists suggest that nasal vowel resonance may still be a surface manifestation of a latent underlying nasal consonant. Although some have attributed this feature to a Celtic substratum influence, there is no definitive consensus on this theory. Over time, these languages have also experienced periods of denasalization, such as in 16th-century French, where nasal consonants were retained in certain contexts, like the feminine form bonne compared to the masculine bon.2