(the triangle over robots indicates that we have abbreviated structure inside this constituent.) Figure 6.5 Tree diagram for
The tree diagram in Figure 6.5 shows this. Our phrase structure rule for NPs, for example, could build NPs that contain a determiner (or DP), a noun, and a PP, but there was no sub-grouping. The key feature of X-bar theory (and the source of its name) arises from the observation that phrases aren’t just a flat structure. Active research in syntax consists of investigating grammatical patterns in languages, and showing how they do (or do not) require specific revisions to current syntactic theories. If we encounter evidence that is inconsistent with this hypothesis, we would revise the theory to account for new data. The assumption that all phrases involve the same structure, and that this is true in all languages, is a hypothesis. The many very fast spaceships carried a lot of valuable cargo. In other words, while the two sentences in (1) are in one sense very different (one has two words, the other has 11), in another sense they have the same structure: both sentences consist of an NP followed by a VP. What this means is that even when a noun or verb-or any other category-doesn’t obviously have any other words in the same phrase as it, it’s still inside an NP or a VP. Every head (X) projects a phrase of the same category (XP).Every phrase (XP) has a head of the same category (X)Īnd this goes the other way as well: all heads (words) project (or “occur inside”) a phrase of their category:.The restriction that all natural languages phrases have heads of the same category is the first limit we’ll put on possible structures in X-bar theory: This rule is weird because it’s a noun phrase that’s missing the noun: we already saw in Section 6.3 is that what makes something a noun phrase is precisely that it has a noun inside it. Weird phrase structure rule: NP –> V (Adj) PP.To see why we might want to constrain what trees are possible, let’s begin by thinking about a type of structure that’s really easy to describe using a phrase structure rule: It’s called that because it introduces an extra layer of structure inside phrases called the “bar level”. In this section we introduce X-bar theory, which aims to make stronger predictions by restricting the shape of possible trees. Constituency tests and phrase structure rules provide a useful starting point for thinking about the structure of possible sentences, but they don’t really start explaining why certain structures are grammatical, or predicting what possible and impossible grammars might look like.
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