Sandhi
Examples of a pro(s)thetic vowel performing external sandhi are known, e.g., in Italian language. Compare: la scuola ("the school") vs. in iscuola ("at school"). It is therefore conjectured that the origins of the Romance prothesis are phonetical ones, rather than grammatical ones, and initially prothesis was for breaking consonant clusters with the preceding word ending in consonant. This hypothesis is corroborated by the absence of prothesis in Romance dialects that had lost their terminal consonants
Read more about this topic: Prothesis (linguistics)