How is the (rare) Locative Plural formed?

Question

If I understood right the Locative is only to be formed in singular (e.g. domi, ruri, ...).

But when it comes to words (especially cities / small islands) that only exist in Plural (e.g. Athenae) we have to form a Locative plural, right?

How is that Locative plural formed?
Is it just always similiar to the Ablative Plural (eg. Athenis)?

Answer 1

Allen & Greenough name locative plural endings for the 1^st, 2^nd and 3^rd declensions. They are all identical to the dative/ablative:

• Athenis at Athens (1^st declension)
• Philippis at Philippi (2^nd declension)
• Trallibus at Tralles (3^rd declension)

For the 4^th and 5^th declensions, no locative plurals appear to be attested (and not many singulars either).

Answer 2

The original locative plural had the *-su or *-oysu ending (i.e., Athenasu), then modified and merged with the ablative.

References:

1. https://en.wikipedia.org/wiki/Locative_case

2. https://www.corsi.univr.it/documenti/OccorrenzaIns/matdid/matdid114728.pdf