From Latin rēgula 'straight stick,' from regere 'to direct' — the same root gives us regal and regiment.
A straight-edged strip used for measuring or drawing straight lines; also, a person who governs.
From Middle English ruler, from Anglo-French ruler 'to rule,' from Latin rēgula 'straight stick, rule,' from regere 'to lead straight, direct.' The measuring instrument and the governing person both derive from the idea of keeping things straight and orderly. Key roots: *h₃reǵ- (Proto-Indo-European: "to move in a straight