c. 1200, "principle or maxim governing conduct, formula to which conduct must be conformed" from Old French riule, Norman reule "rule, custom, (religious) order" (in Modern French partially re-Latinized as règle), from Vulgar Latin *regula, from Latin regula "straight stick, bar, ruler;" figuratively "a pattern, a model," related to regere "to rule, straighten, guide" (from PIE root *reg- "move in a straight line," with derivatives meaning "to direct in a straight line," thus "to lead, rule"). Replaced Old English wealdan.
Meaning "regulation governing play of a game, etc." is from 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 or measuring" is recorded from mid-14c. Typography sense is attested from 1680s.
c. 1200, "to control, guide, direct," from Old French riuler "impose rule," from Latin regulare "to control by rule, direct," from Latin regula "rule, straight piece of wood," from PIE root *reg- "move in a straight line," with derivatives meaning "to direct in a straight line," thus "to lead, rule." Legal sense "establish by decision" is recorded from early 15c. Meaning "mark with lines" is from 1590s. Meaning "to dominate, prevail" is from 1874. "Rule Brittania," patriotic song, is from 1740. Related: Ruled; ruling.