From Proto-Baltic *platús, from *pl̥th₂us < *pleth₂- (“flat”). In Latvian, former u-stem adjectives were assimilated into other classes; *platus gave rise to both an o-stem and a yo-stem variant which later on became independent words, plats and plašs, with different semantic nuances (compare also dobs and dobjš, or ass and ašs); this separation began in the 18th century but became complete only in the 1870s. Cognates include Lithuanian platùs, Old Prussian plat- (from a placename, Platmedyen, where median = “forest”), Sanskrit पृथु (pṛthu, “broad, wide, great, powerful”), Ancient Greek πλατύς (platús, “broad, wide, flat, smooth”).