ruler n.

c. 1200, "to control, guide, direct, make conform to a pattern," 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."

The legal sense "establish by decision, lay down authoritatively" is recorded from early 15c. The meaning "mark with parallel straight lines" (with or as with the aid of a ruler) is from 1590s. The slang intransitive sense of "dominate all" is by 1975. "Rule Britannia," patriotic song, is from 1740. Related: Ruled; ruling.

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").

By mid-14c. as "control, government, sway, dominion." The meaning "regulation governing play of a game, etc." is from 1690s; the phrase rules of the game is by 1787. To bend the rules "interpret leniently, overlook infringement" is by 1680s.

The meaning "strip with a straight edge used for making straight lines or measuring" is from mid-14c. Typography sense of "thin strip cut type-high and used for printing continuous lines" is attested from 1680s. Rule of law "supremacy of impartial and well-defined laws to any individual's power" is from 1883. Rule of the road in reference to the fixed customs, formerly much varying from country to country, which regulate the sides to be taken by vehicles in passing each other, is by 1805.

The rule of the road is a paradox quite,

In driving your carriage along,

If you keep to the left you are sure to go right,

If you keep to the right you go wrong.

[Horne Tooke, "Diversions of Purley," 1805]