Why is the ninth month called September?
late 14c., "draw up a line of battle," from Old French arengier (12c.), from a- "to" (see ad-) + rangier "set in a row" (Modern French ranger), from rang "rank," from Frankish *hring (see rank (n.)).
A rare word until the meaning generalized to "to place things in order" c.1780-1800. Musical sense of "adapt for other instruments or voices" is from 1808. Related: Arranged; arranging. Arranged marriage attested from 1854.