Origin and history of alternately

"following each other by turns, reciprocal," 1510s, from Latin alternatus "one after the other," past participle of alternare "to do first one thing then the other; exchange parts," from alternus "one after the other, alternate, in turns, reciprocal," from alter "the other" (see alter).

Alternate means "by turns;" alternative means "offering a choice." Both imply two kinds or things. Alternation is the process of two things following one another regularly by turns (as night and day); an alternative is a choice of two things, the acceptance of one implying the rejection of the other. Related: Alternacy.

common adverbial suffix, forming, from adjectives, adverbs signifying "in a manner denoted by" the adjective; Middle English -li, from Old English -lice, from Proto-Germanic *-liko- (source also of Old Frisian -like, Old Saxon -liko, Dutch -lijk, Old High German -licho, German -lich, Old Norse -liga, Gothic -leiko). See -ly (1). It is cognate with lich, and identical with like (adj.).

Weekley notes as "curious" that Germanic uses a word essentially meaning "body" for the adverbial formation, while Romanic uses one meaning "mind" (as in French constamment from Latin constanti mente). The modern English form emerged in late Middle English, probably from influence of Old Norse -liga.