80. The Locative form for nouns of the 3rd declension ends in the singular in -ī or -ē, in the plural in -ibus.
rūrī in the country
Carthāginī or Carthāgine at Carthage
Trallibus at Tralles1
1. The Indo-European locative singular ended in -ĭ, which became -ĕ in Latin. Thus the Latin ablative in -e is, historically considered, a locative. The Latin ablative in -ī (from -īd) was an analogical formation (cf. -ā from -ād, -ō from -ōd), properly belonging to i-stems. With names of towns and a few other words, a locative function was ascribed to forms in -ī (as, Carthāginī), partly on the analogy of the real locative of o-stems (as, Corinthī, § 49.a); but forms in -ĕ also survived in this use. The plural -bus is properly dative or ablative, but in forms like Trallibus it has a locative function. Cf. Philippīs (§ 49.a), in which the ending -īs is, historically considered, either locative, or instrumental, or both, and Athēnīs (§ 43.c), in which the ending is formed on the analogy of o-stems.