This article investigates the Final-over-Final Constraint (FOFC): a head-initial category cannot be the immediate structural complement of a head-final category within the same extended projection. This universal cannot be formulated without reference to the kind of hierarchical structure generated by standard models of phrase structure. First, we document the empirical evidence: logically possible but crosslinguistically unattested combinations of head-final and head-initial orders. Second, we propose a theory, based on a version of Kayne’s (1994) Linear Correspondence Axiom, where FOFC is an effect of the distribution of a movement-triggering feature in extended projections, subject to Relativized Minimality.