Moreover, the X-bar rules in (1) rule out another possibility if we assume that these are the only rules determining structure. While it might not make much sense to have a phrase with a head of a different category, the idea of a phrase that simply lacks a head is not so absurd. There is a traditional distinction made between endocentric and exocentric language elements. An endocentric phrase gets its properties from an element that it contains and hence this element can function by itself as the whole phrase. For example:
An exocentric phrase on the other hand contains no element that can have the same function as the whole phrase and so appears to have properties that are independent from the elements it contains. A standard example is:
Therefore it would appear that clauses are exocentric constructions, having no heads, and as such stand outside of the X-bar system. Later in this book, we will challenge this traditional conclusion and claim that clauses do indeed have heads, though the head is neither the subject nor the VP. From this perspective, X-bar theory is a completely general theory applying to all constructions of the language and given that X-bar theory consists of just three rules it does indeed seem that I-language principles are a lot simpler than observation of E-language phenomena would tend to suggest.
6.1 The structure of IP
Note that apart from complementisers, which we will discuss in the next chapter, and adverbials, which we will discuss at the end of the present chapter, this structure accounts for all elements of the clause. Specifically we have a subject position, an inflection and a VP predicate: the three obligatory parts of the clause. It seems reasonable to claim therefore that the IP IS the clause. This point of view addresses an issue raised in chapter 3 concerning the exocentric nature of the clause. There we discussed reasons for not considering the subject or the VP as the heads of the sentence as they do not seem to have the right properties of a head. Traditionally therefore it has been assumed that clauses are headless.
6.1 The structure of IP
To see the advantage of this analysis, consider what happens if we do not assume that the inflectional element is the head of the clause. English is often described as an SVO language, based on a way of classifying languages in accordance with the ‘typical’ ordering of the major elements of the sentence (subject, verb and object). Without X-bar theory and the notions of head, complement and specifier, however, this is just a description of the facts which tells us nothing beyond what can already be observed. Assuming X-bar theory we have a way of accounting for word order patterns by using general statements about the relationships between elements in an X-bar structure and so this is a step in the right direction. However, if we do not assume that the inflection is a head, it is not easy to think of how we can use X-bar generalisations to account for the basic word order of English. This is especially so if we take the traditional view that sentences are exocentric and therefore stand outside of the set of facts that X-bar theory can account for. Only if we assume that sentences are endocentric can X-bar generalisations be used to account for word order facts concerning sentences.