A thin plate of brass or other metal, of the same height as the type, and used for printing lines, as between columns on the same page, or in tabular work.
From Middle English reule, rewle, rule, borrowed from Old French riule, reule, itself an early semi-learned borrowing from Latin regula ("straight stick, bar, ruler, pattern"), from regō, ultimately from Proto-Indo-European *h₃réǵeti ("to straighten; right"), from the root *h₃reǵ-; see regent.
From Middle English rulen, borrowed from Old French riuler, from Latin regulāre ("to regulate, rule"), from regula; see regular and regulate.