Quote Originally Posted by chaika
No, it's dual, as in when you are talking about two things. The dual had its own endings that were not the same as the singular or the plural. For example, here are four words, for type, horse, year, and sea,

Nom./Acc./Vocative рода, коня, лѣтѣ, мори
Gen./Locative роду, коню, лѣту, морю
Dat./Instr. родома, конема, лѣтома, морема

From Wiki.


The seas are drying up.
If you are talking about several seas you would say моря, but if you meant only two you would say мори.

Aren't you glad you don't have to memorize a whole nother set of endings (adjectives and verbs had endings for the dual as well).

The dual is a common IndoEuropean grammatical feature found in all IE languages, at least in their older forms -- I don't know whether it is still alive anywhere, but I wouldn't be surprised. My Serbocroatian and Bulgarian is failing me here.
Slovenian is the only Slavic language that retains a full dual number.

Remnants of the dual number exist in Ukrainian more than Russian. E.g.очима is the instrumental plural of eyes (nom. sig. око), [the normal, regular instrumental plural ending is -ами, like in Russian] which is similar to the:
Dat./Instr. родома, конема, лѣтома, морема chaika posted.

Also, I sort of mentioned this before, in Russian:
сто
двести (две сти) - сти was the dual number of сто.
триста - ста is the genitive singular of сто.
пятьсот - сот is the genitive plural of сто.