12.3.7 Coordination

The major phrase types discussed here can be conjoined with conjunctions like and, or, and but to form larger constructions of the same type. For example a coordinate noun phrase can consist of two other noun phrases separated by a conjunction:

Please repeat $ [_{NP}\ [_{NP}\ the\ flights]\ and\ [_{NP}\ the\ costs]] $

I need to know $ [_{NP}\ [_{NP} the aircraft]\ and\ [_{NP} the flight number]] $

Here's a rule that allows these structures:

$$ NP\ \to\ NP\ and\ NP $$

Note that the ability to form coordinate phrases via conjunctions is often used as a test for constituency. Consider the following examples which differ from the ones given above in that they lack the second determiner.

Please repeat the [Nom [Nom flights] and [Nom costs]]

I need to know the [Nom [Nom aircraft] and [Nom flight number]]

The fact that these phrases can be conjoined is evidence for the presence of the underlying Nominal constituent we have been making use of. Here's a new rule for this:

Nominal $ \rightarrow $ Nominal and Nominal

The following examples illustrate conjunctions involving VPs and Ss.

What flights do you have [VP [VP leaving Denver] and [VP arriving in San Francisco]]

[s [s I'm interested in a flight from Dallas to Washington] and [s I'm also interested in going to Baltimore]]

The rules for VP and S conjunctions mirror the NP one given above.

$$ VP\ \to\ VP\ and\ VP $$

$$ S\;\rightarrow\;S~a n d~S $$

Since all the major phrase types can be conjoined in this fashion it is also possible to represent this conjunction fact more generally; a number of grammar formalisms such as (Gazdar et al., 1985) do this via metarules such as the following:

$$ x\to x and x $$

This metarule simply states that any non-terminal can be conjoined with the same non-terminal to yield a constituent of the same type. Of course, the variable X must be designated as a variable that stands for any non-terminal rather than a non-terminal itself.

12.4 TREEBANKS

Context-free grammar rules of the type that we have explored so far in this chapter can be used, in principle, to assign a parse tree to any sentence. This means that it is possible to build a corpus in which every sentence is syntactically annotated with a parse tree. Such a syntactically annotated corpus is called a \textit{treebank}. Treebanks play an important role in parsing, as we will see in Ch. 13, and in various empirical investigations of syntactic phenomena.

A wide variety of treebanks have been created, generally by using parsers (of the sort described in the next two chapters) to automatically parse each sentence, and then using humans (linguists) to hand-correct the parses. The Penn Treebank project (whose POS tagset we introduced in Ch. 5) has produced treebanks from the Brown, Switchboard, ATIS, and Wall Street Journal corpora of English, as well as treebanks in Arabic and Chinese. Other treebanks include the Prague Dependency Treebank for Czech, the Negra treebank for German, and the Susanne treebank for English.