Assyriologists and Semitists have assumed that the attributive masculine plural morpheme -ūt found in Akkadian is a secondary development in analogy with the feminine plural -āt. In this paper I suggest that this morpheme should be reconstructed to Proto-Semitic. My arguments are based on some attributive pronouns in West Semitic and the existence of a distinction between attributive and predicative plural morphology in both branches of Semitic.