early 13c., "principle or maxim governing conduct," from O.Fr. riule, from V.L. *regula, from L. regula "straight stick, bar, ruler, pattern," related to regere "to rule, straighten, guide" (see right). Replaced O.E. wealdan. Meaning "regulation governing play of a game, etc."
is from 1690s. Phrase rule of thumb first attested 1690s. Rule of law "supremacy of impartial and well-defined laws to any individual's power" is from 1883. Meaning "Strip used for making straight lines" is recorded from mid-14c. Typography sense is attested from 1680s.
early 13c., "to control, guide, direct," from O.Fr. riuler, from L. regulare. Legal sense is recorded from 1425 (ruling "judicial decision" is from 1560). Ruler "one who rules" is recorded from late 14c.; meaning "strip used for making straight lines" is c.1400 (see rule (n.)).