Languages of the first category, belonging to Western Romance, generally employ a plural suffix morpheme -s. Languages of the second category, belonging to Italo-Dalmatian and Eastern Romance, form the plural by changing the final vowel of the singular form, or suffixing a new vowel to it.
There are various hypotheses about how these systems—especially the second—emerged historically from the declension patterns of Vulgar Latin, and this remains an area of much debate and controversy amongst scholars of Romance.
The second group, consisting of the Romance languages south or east of the La Spezia–Rimini Line (i.e. Italo-Dalmatian and Eastern Romance), involves changing (or adding) the final vowel; for example: The following table illustrates the singular and plural forms of the first, second, and third declensions in Classical Latin.
On the other hand, 3rd declension nouns and adjectives have -es in both nominative and accusative, however, so the -s plural for these words could derive from either case form.
Additionally, Old French feminine plurals end in -es in both the nominative and the oblique (accusative); this may be evidence in favour of a more general Proto-Romance replacement of -AE by -ĀS.
Sardinian duncas suggests Proto-Romance *DUNQUAS, with dunque the expected outcome (even down to the unusual qu preceding e) if -AS > e. The "accusative" theory essentially suggests: The first of these changes is almost certain, given examples like tu stai 'you stand' < TŪ STĀS; Italian crai 'tomorrow' (archaic, literary or regional) < CRĀS; tu sei 'you are' < TŪ *SES; sei 'six' < SEX (probably Proto-Italian *sess).