The set of person and number features necessary to characterize the pronominal paradigm the world’s languages is highly constrained, and their interaction is demonstrably systematic. We develop a geometric representation of morphosyntactic features which provides a principled explanation for the observed restrictions on these paradigms. The organization of this geometry represents the grammaticalization of fundamental cognitive categories, such as reference, plurality and taxonomy. We motivate the geometry through the analysis of pronoun paradigms in a broad range of genetically distinct languages.