X-bar TheoryNote that the X' and the phrase share the same categorial status (X) and so if X' is
P' XP will be PP, etc. As X' is the same category as the head, it follows that the whole
phrase will be of the same category as the head. In this way, the head of the phrase
determines the phrase’s category.
The property of sharing category between the head, the X' and the phrase is called
projection. We say that the head projects its categorial status to the X' and ultimately
to the XP. If we put the two parts of the structure together, we can more clearly see
how projection works:
(7) VP
DP V'
the children V PP
fall over
The line of projection proceeds from the head, via the X' to the phrase thus ensuring
that phrases and heads match.
The meaning of the ‘bar’ can be seen in terms of the notion of projection. We can
imagine a phrase as a three-floored building, with a ground floor, a first floor and a top
floor. On the ground floor we have the head, which is not built on top of anything – it
is an unprojected element. Often heads are called zero level projections, to indicate
that they are not projected from anything. This can be represented as X^0.
Above the head, we have the X', the first projection of the head. The bar then
indicates the projection level of the constituent: X' is one projection level above X^0.
On the top floor we have the phrase, XP. This is the highest level projected from
the head and hence it is called the maximal projection. Another way of representing
the maximal projection is X'', an X with two bars (pronounced ‘X double bar’), with
the bars again representing the projection level. It seems that all phrases project to two
levels and so we will not entertain the possibility of X''', or X'''', etc. Typically we will
maintain the custom of representing the maximal projection as XP.
1.2 Endocentricity
An obvious consequence of the notion of projection is that we will never get a phrase
of one category with a head of another. While this might seem a slightly perverse
situation to want to prevent in the first place (why would verb phrases be headed by
anything other than a verb?), it is certainly a logical possibility that there could be
phrases of category X which do not contain a word of category X. For example the
traditional view that preposition phrases can function adverbially could be captured
under the following assumption:
(8) AP Æ P DP
In other words, a preposition phrase which behaves as an adverbial phrase is an adverb
phrase headed by the preposition. Clearly this is something that would not be allowed
by the X-bar rules in (1). Evidence favours the X-bar perspective and there is no
reason to believe that just because something functions adverbially it is categorially the