The Allen and Greenough is still under construction;
so some links may not work quite the way you would expect.
The Locative Case.
80. The Locative form for nouns of the third
declension end in the singular in -í or -e, in the plural in -ibus: as, rúrí, in the country;
Cartháginí or Carthágine, at Carthage; Trallibus, at Tralles.[The
Indo-European locative singular ended in -í, which became -e in Latin. Thus the Latin ablative in
-é is, historically considered,
a locative. The Latin ablative -í (from -íd) was an analogical formation (cf. -á from -ád, -ó from -ód), properly belonging to i-stems. With names of towns and a few other
words, a locative function was ascribed to forms in -í (as, Cartháginí), partly on the
analogy of the real locative o-stems
(as, Corinthí, § 49. a); but forms in -e also
survived in this use. The plural -bus
is properly dative or ablative, but in forms like Trallibus it has a locative function. Cf.
Philippís (§ 49. a), in which the ending -ís is, historically considered, either
locative, or instrumental, or both, and Athénís (§ 43. c),
in which the ending is formed on the analogy of o-stems.]