// n. (also X-bar theory) A system designed to formalize the traditional notion ‘head of a construction’ and to constrain the range of possible phrase structure rules. The heart of the system is the recognition that syntactic categories are projected from lexical items which are their lexical heads, so that the category Noun Phrase, for example, is analysed as differing from the category Noun primarily in respect of the value assigned to a feature conventionally called [BAR]: Noun is [NOUN] [BAR 0], while NP is [NOUN] [BAR 2] (variously abbreviated as or N2) in most current versions of the system), though some earlier proposals suggested [NOUN] [BAR 3]). The system obliges us to regard syntactic categories as complex symbols.
Most versions of the system incorporate some kind of requirement that a category should have a head, a requirement which, in principle at least, greatly reduces the number of phrase structure rules which can be written. The X-bar system was first suggested, in a somewhat inexplicit form, by Zellig Harris (1951); it was revived by Noam Chomsky (1970), and developed by Joseph Emonds (1976) and more particularly by Ray Jackendoff (1977). Almost all contemporary theories of grammar incorporate some version of the system; that associated with GPSG, as presented in Gazdar et al. (1985), is particularly well articulated. A critical review of X-bar practice is given in Kornai and Pullum (1990); these authors identify six properties which are commonly associated with the X-bar system: lexicality,succession,uniformity,maximality,centrality and optionality.
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/ n. (also X-bar theory) A system designed to formalize the traditional notion ‘head of a construction’ and to constrain the range of possible phrase structure rules. The heart of the system is the recognition that syntactic categories are projected from lexical items which are their lexical heads, so that the category Noun Phrase, for example, is analysed as differing from the category Noun primarily in respect of the value assigned to a feature conventionally called [BAR]: Noun is [NOUN] [BAR 0], while NP is [NOUN] [BAR 2] (variously abbreviated as 
or N2) in most current versions of the system), though some earlier proposals suggested [NOUN] [BAR 3]). The system obliges us to regard syntactic categories as 
