↑ Hellquist, Elof. 1922. Svensk etymologisk ordbok./
↑ Fick, Falk and Torp. 1909. Wortschatz der Germanischen Spracheinheit/
Etymology
From Proto-Germanic*rehtaz, whence also Old English riht, Old High German reht. Ultimately from Proto-Indo-European*h₃reǵtós. The noun réttr, however, is from a Proto-Germanic u-stem (*rehtuz).[1][2]