W. M. Lindsay, Syntax of Plautus

Locative

Comparative Philology has corrected the old notion that -ī was in all Declensions the Locative suffix (e.g. Romai, Corinthi, Carthagini), and has shown that in Ā-stems (1 Declension) the suffix was -ai, a diphthong (while the Genitive suffix was disyllabic -āī), in O-stems (2 Declension) -oi (cf. Gk. οἴκοι) which became -ei, and later (after Plautus' time) -ī, in Consonant-stems (part of 3 Declension) -ĭ, which became -ĕ. This Consonant-stem Locative was used in Latin as Ablative, e.g. Carthaginĕ, patrĕ, in Greek as Dative, e.g. πατρί. Instead of this Ablative-Locative -ĕ in Consonant-stems we find occasionally -ī in Plautus, e.g. militi, which seems to be the I-stem Ablative (originally -īd), e.g. navī, classī. Just as the Consonant-stem suffix -ĕ was often used in I-stems, e.g. navĕ, classĕ, and (in Plautus) marĕ, so the I-stem suffix -ī(d) found its way into Consonant-stems. If this be the true explanation, Carthagini, mani, etc., and in Plautus Accherunti ‘in the lower world,’ e.g. Capt. 998, are Ablatives, not Locatives.