In many languages the definite article cannot occur when a possessive phrase is present in the NP (e.g. English the my book, John's the book). I argue that these patterns can be understood in terms of economic motivation because possessed NPs are very likely to be definite. A structural explanation in terms of a unique determiner position is insufficient to account for the full range of attested crosslinguistic patterns, and the universal generalizations that do seem to be valid can be derived from the economy-based explanation. Finally I show how the performance motivation of economy creates the competence pattern in diachronic change.