In Latin, the functions of the locative case were mostly absorbed by the
ablative, but a separate locative is found in a few words.
The Latin locative case applies only to the names of cities and small
islands and to a few other isolated words. The Romans considered all
Mediterranean islands to be small except for Sicily, Sardinia, Corsica,
Crete, and Cyprus. Britannia was also considered to be a "large island."
There are a few nouns that use the locative instead of a preposition:
domus becomes domī (at home), rūs becomes rūrī (in the country), humus
becomes humī (on the ground), militia becomes militiae (in military
service, in the field), and focus becomes focī (at the hearth; at the
center of the community).
For singular first and second declension, the locative is identical to
the genitive singular form, and for the singular third declension the
locative is identical to the ablative singular form. For plural nouns of
all declensions, the locative is also identical to the ablative form.
The few fourth and fifth declension place-name words would also use the
ablative form for locative case.
In archaic times, the locative singular of third declension nouns was
actually interchangeable between ablative and dative forms, but in the
Augustan Period the use of the ablative form became fixed. Therefore,
both forms "rūrī" and "rūre" may be encountered.
The first declension locative is by far the most common, because so many
Roman place names were first declension: mostly singular (Roma, Rome;
Hibernia, Ireland; etc., and therefore Romae, at Rome; Hiberniae, at
Ireland), but some plural (Athenae, Athens; Cumae, Cuma etc., with
Athenis, at Athens; Cumis, at Cumae). But there are a number of second
declension names that would have locatives, too (Brundisium, Brindisi;
Eboracum, York; with locatives Brundisiī, at Brindisi; Eboraci, at York,
etc.)
(
http://en.wikipedia.org/wiki/Locative_case)
Romae omnia venalia esse
(Sallust)
Ed