<P>It is standardly assumed that Universal Grammar (UG) allows a given hierarchicalrepresentation to be associated with more than one linear order. For example, English and Japanesephrases consisting of a verb and its complement are thought of as symmetrical to one another,differing only in linear order. The Antisymmetry of Syntax proposes a restrictive theory of wordorder and phrase structure that denies this assumption. According to this theory, phrase structurealways completely determines linear order, so that if two phrases differ in linear order, they mustalso differ in hierarchical structure.More specifically, Richard Kayne shows that asymmetricc-command invariably maps into linear precedence. From this follows, with few further hypotheses, ahighly specific theory of word order in UG: that complement positions must always follow theirassociated head, and that specifiers and adjoined elements must always precede the phrase that theyare sister to. A further result is that standard X-bar theory is not a primitive component of UG.Rather, X-bar theory expresses a set of antisymmetric properties of phrase structure. Thisantisymmetry is inherited from the more basic antisymmetry of linear order.Linguistic InquiryMonograph No. 25</P>