This book presents evidence for a universal word order constraint, the Final-over-Final Constraint (FOFC), and discusses the theoretical implications of this phenomenon. FOFC is a syntactic condition that disallows structures where a head-initial phrase is contained in a head-final phrase in the same extended projection/domain. The authors argue that FOFC is a linguistic universal, not just a strong tendency, and not a constraint on processing. They discuss the effects of the universal in various domains, including the noun phrase, the adjective phrase, the verb phrase, and the clause. The book draws on data from a wide range of languages, including Hindi, Turkish, Basque, Finnish, Afrikaans, German, Hungarian, French, English, Italian, Romanian, Arabic, Hebrew, Mandarin, Pontic Greek, Bagirmi, Dholuo, and Thai. FOFC, the authors argue, is important because it is the only known example of a word order asymmetry pertaining to the order of heads. As such, it has significant repercussions for theories connecting the narrow syntax to linear order.