In historical linguistics, Weise's law describes the loss of palatal quality that some consonants undergo in specific contexts in the Proto-Indo-European language (PIE). Specifically, when the palatovelar consonants *ḱ *ǵ *ǵʰ are followed by *r, they lose their palatal quality, leading to a loss in distinction between them and the plain velar consonants *k *g *gʰ. Some exceptions exist, such as when the *r is followed by *i or when the palatal form is restored by analogy with related words. Although this sound change is most prominent in the satem languages, the change probably occurred prior to the centum–satem division, based on an earlier sound change which affected the distribution of PIE *u and *r.
The law is named after the German linguist Oskar Weise, who postulated it in 1881 in order to reconcile cognates in Ancient Greek and Sanskrit.