I have only studied the very basics of Greek years ago, so the question might be simple, but I couldn't find a clear summary anywhere. Attic Greek has a dual number alongside singular and plural. When was this number used? Was it used whenever there were two items but never otherwise, or just rarely in some archaic expressions? Would it always be safe to use the plural instead or are there situations where the dual is required?
I hope someone posts a more thorough answer, but as a starting point, my impression of Attic usage (at least as exemplified by Plato) is as follows. First, the dual is always optional, both in nouns and verbs; it doesn't seem ever to be obligatory and can always be replaced with a plural. On the other hand, it isn't terribly rare, archaic, or limited to specific referents or registers, either. Plato's speakers seem to use it fairly productively, if inconsistently. Overall it looks like in Plato's time the dual was still part of the living everyday language, but was in the process of getting ousted by the plural.
In Homer, the dual seems to be freely used whenever two items are mentioned, but this became rarer as time went on. It's interesting that in Hebrew (which also had a dual), the use of the dual was basically confined to items that naturally occurred in pairs, e.g. Heb. yadayim (a dual form) from yad "hand", but in the Iliad, at least, we have such forms as Αἴαντε, "the two Ajaxes" (in spite of the fact that they merely shared a name). But, even in Homer, the plural always seems to be a possible alternative to the dual.