Used antithetically in contrast with the other of two persons or things. Chiefly in the one…the other; also poetic without the (rare). the one and…

The earlier word for both, pron. adv.adj. Modifying or in apposition to a pronoun.

Each of the two. Without construction. Also in genitive, as either's. Obsolete (archaic in later use).

With reference to two things, people, or groups previously specified: the two (without exception); the one as well as the other.

Each of the two; = either, pron. 1.