We noted in the previous section that by factoring subcategorization information out of the main category label, we could express more generalizations about properties of verbs. Another property of this kind is the following: expressions of category V are heads of phrases of category VP. Similarly, Ns are heads of NPs, As (i.e., adjectives) are heads of APs, and Ps (i.e., prepositions) are heads of PPs. Not all phrases have heads—for example, it is standard to say that coordinate phrases (e.g., the book and the bell) lack heads. Nevertheless, we would like our grammar formalism to express the parent/head-child relation where it holds. At present, V and VP are just atomic symbols, and we need to find a way to relate them using features (as we did earlier to relate IV and TV).
X-bar syntax addresses this issue by abstracting out the notion of phrasal level. It is usual to recognize three such levels. If N represents the lexical level, then N' represents the next level up, corresponding to the more traditional category Nom, and N'' represents the phrasal level, corresponding to the category NP. (36a) illustrates a representative structure, while (36b) is the more conventional counterpart.
The head of the structure (36a) is N, and N' and N'' are called (phrasal) projections of N. N'' is the maximal projection, and N is sometimes called the zero projection. One of the central claims of X-bar syntax is that all constituents share a structural similarity. Using X as a variable over N, V, A, and P, we say that directly subcategorized complements of a lexical head X are always placed as siblings of the head, whereas adjuncts are placed as siblings of the intermediate category, X'. Thus, the configuration of the two P'' adjuncts in (37) contrasts with that of the complement P'' in (36a).
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