modus

modus, modi is a second declension masculine noun, entirely regular. Its first sense is a measure, then the limit that a measure sets, then the manner in which a thing is done, and Horace's est modus in rebus uses the first two at once: there is a proper measure in things, and therefore a limit.

Most of its work is done in the ablative of manner: hoc modo "in this way", eo modo "in that way", nullo modo "by no means", quo modo "how", which fuses into quomodo and comes down into Spanish como and French comment. The adverb modo, "only, just now", is this word's ablative frozen solid, and has its own page. English "mode", "modest", "moderate", "modify" and the grammatical "mood" all descend from it.