Proto-Indo-European root meaning "to line up."

It forms all or part of: assert; assertion; assort; consort; desert (v.) "to leave one's duty;" desertion; dissertation; ensorcell; exert; exsert; insert; seriatim; seriation; series; sermon; serried; sorcerer; sorcery; sort.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit sarat- "thread;" Greek eirein "to fasten together in rows;" Latin serere "to join, link, bind together," series "row, chain, series, sequence, succession;" Gothic sarwa (plural) "armor, arms;" Old Norse sörve "necklace of stringed pearls;" Old Irish sernaid "he joins together;" Welsh ystret "a row."

1650s, in military language, "a rapid regrouping for renewed action after a repulse," from rally (v.1). Sense of "a mass meeting to stir enthusiasm" is attested by 1840, American English. Sense of "gathering of automobile enthusiasts" is from 1932, from French rallye, itself from the English noun. Sports sense of "long series of hits from one side to the other" in tennis, etc., is from 1881, earlier "series of back-and-forth blows in a boxing match" (1825).

[standard of measure or estimation] late 14c., "series of registering marks; marks laid down to determine distance along a line," (in Chaucer's description of the astrolabe), from Latin scala "ladder, flight of stairs," from *scansla, from stem of scandere "to climb" (see scan (v.)). 

The noun in the classical Latin sense is rare, though Middle English had it as "ladder used in sieges" (c. 1400). The meaning "succession or series of steps ascending or descending" is from c. 1600; that of "standard for estimation" (large scale, small scale, etc.) is from 1620s.

The musical sense of "definite and standard series of tones within a certain range," typically an octave (1590s), and the meaning "proportion of a representation to the actual object" (1660s) are via Italian scala, from Latin scala.

"next to the last, immediately proceeding that member of a series which is the last," 1670s, from penultima (n.) on model of proximate. Earlier was penultim (mid-15c.), from Old French penultime.

type of bagpipe music consisting of a series of variations on a theme, 1719, from Gaelic piobaireachd, literally "piper's art," from piobair "a piper" (from piob "pipe," an English loan word; see pipe (n.1)) + -achd, suffix denoting function.

1660s, "bond, link, interdependence between members of a series or group; means of communication," from Latin nexus "that which ties or binds together," past participle of nectere "to bind," from PIE root *ned- "to bind, tie."