We've seen that a transitive verb combines with two arguments: the subject, which frequently
expresses the agent, although not necessarily, and the object, which again not necessarily
but frequently expresses the theme. And we've also seen that those arguments are combined
with the verb in a particular order, so the verb combines first with the object and forms
a constituent with that, and then the result of that combines with the subject to form
a larger constituent. And, we've also seen that we would want to associate or express
these selectional properties of the verb by representing in the lexicon an elementary
tree for the verb that shows these positions for the two arguments.
Further, as we've already discussed, the properties of a phrase depend on the properties of the
head. So in this case, since the head is a verb, it projects a verb phrase. So in this
more elaborate structure, you can see that the lexical category of the head, the V, projects
first to this larger constituent, the V-bar, and then to an even larger constituent, VP.
We've seen phrases of course that have heads other than V, so we can have phrases headed
by prepositions: PPs, or post-positions also. We've seen that you can have phrases headed
by adjectives also, so you have adjective phrases.
We can hypothesize that phrases might all have a similar kind of structure, the kind
of structure we've just seen for the VP. That hypothesis is sometimes called the X-bar schema.
So according to the X-bar schema, all phrases have a similar structure to what we've just
seen for the VP. One way to represent that is to take a variable to stand for the actual
lexical category. So, the head X, which could be V or A, will combine with one phrase and
form a constituent X-bar. So when V combines with its complement, you get a V-bar, if an
adjective combined with a complement you would get an A-bar. And then, the resulting phrase
combines with another phrase, so the V-bar combines with another phrase and you would
get the VP or more generally an X-bar combines with another phrase and you get the XP.
So those two positions for phrases that are inside this X-bar schema have names just for
these positions. So the complement of X is sister to the head, and then the phrase which
is higher up in the structure, which is the sister to the x-bar, is called the specifier.
So, just to summarize, a head (an X), projects two levels of phrase: it projects to an X-bar,
which can contain the head and its... and the complement of the head, and it can project
further to the XP, which would contain also the specifier.
So far, we've seen this for verb phrases, but you could come up with a possible hypothesis
for the structure of noun phrases along the kind of lines that people have already been
considering in class, where a noun phrase would also have a head, N in this case, which
would project an N-bar, which would contain also the complement of the... the nominal
head, and then it would project further to an NP. So that's one possible structure for
nominal phrases. As we've already discussed, in fact there's an alternative way of looking
at nominal phrases which is to consider that rather than being projections of the noun,
they're projections of the determiner. We'll be looking at that in more detail later. In
that case, the structure of a nominal phrase, we would actually want to say that it's a
DP, because it's projected from the D, rather than NP, projected from the N, and in the
other diagrams that you'll see, I've actually assumed that's the case, so you'll see DPs
everywhere, but that's not important for the point here, we're going to look at DPs and
NPs later. The point is just that this X-bar schema is intended as a general scheme for
phrases of different categories.
As it stands, it looks like that schema is not going to allow for all of the variety
we've already seen. In particular, it seems in some cases it's going to give us too much
structure. So, for example, although a transitive verb takes a complement as well as a subject,
we've seen plenty of cases of intransitive verbs, which have subjects but no objects,
so we'll assume that that complement position is actually optional and the specifier position,
we can assume, also may be optional. And so it will depend on the particular properties
of the head whether it occurs with a complement and whether it occurs with a specifier.
There are also cases where it seems that the structure that we get from the X-bar schema
isn't actually enough, that we don't have enough positions within the phrase if that's
all there is. So two cases you could already think of are ditransitive verbs, where you
actually have three arguments to accommodate, and also all these cases of modifiers that
we've seen, where it seems you have an, ah, an unlimited number of modifiers that can
be included in a phrase. We'll come back to ditransitives in another class, but we'll
get back to modifiers very shortly.