Relative Clauses

Relative clauses are NP modifiers, which involve extraction of an argument or an adjunct. The NP head (the portion of the NP being modified by the relative clause) is not directly related to the extracted element. For example in ((157)), the person is the head NP and is modified by the relative clause whose mother $\epsilon$ likes Chris. The person is not interpreted as the subject of the relative clause which is missing an overt subject. In other cases, such as ((158)), the relationship between the head NP export exhibitions may seem to be more direct but even there we assume that there are two independent relationships: one between the entire relative clause and the NP it modifies, and another between the extracted element and its trace. The extracted element may be an overt wh-phrase as in ((157)) or a covert element as in ((158)).

(156)0(156

(157)

the person whose mother likes Chris  (157)0(157

(158)

export exhibitions that included high-tech items 

Relative clauses are represented in the English XTAG grammar by auxiliary trees that adjoin to NP's. These trees are anchored by the verb in the clause and appear in the appropriate tree families for the various verb subcategorizations. Within a tree family there will be groups of relative clause trees based on the declarative tree and each passive tree. Within each of these groups, there is a separate relative clause tree corresponding to each possible argument that can be extracted from the clause. There is no relationship between the extracted position and the head NP. The relationship between the relative clause and the head NP is treated as a semantic relationship which will be provided by any reasonable compositional theory. The relationship between the extracted element (which can be covert) is captured by co-indexing the <trace> features of the extracted NP and the NP_w node in the relative clause tree. If for example, it is NP₀ that is extracted, we have the following feature equations:
NP_w.t:$\langle$ trace $\rangle =$NP₀.t:$\langle$ trace $\rangle$
NP_w.t:$\langle$ case $\rangle =$NP₀.t:$\langle$ case $\rangle$
NP_w.t:$\langle$ agr $\rangle =$NP₀.t:$\langle$ agr $\rangle$ ^14.1 Representative examples from the transitive tree family are shown with a relevant subset of their features in Figures 14.1(a) and 14.1(b). Figure 14.1(a) involves a relative clause with a covert extracted element, while figure 14.1(b) involves a relative clause with an overt wh-phrase.^14.2

    

Figure: Relative clause trees in the transitive tree family: $\beta$Nc1nx0Vnx1 (a) and $\beta$N0nx0Vnx1 (b)

(a)	(b)

The above analysis is essentially identical to the GB analysis of relative clauses. One aspect of its implementation is that an covert +<wh> NP and a covert Comp have to be introduced. See ((159)) and ((160)) for example.

(158)0(158

(159)

export exhibitions [ [_NP<<13891>>w$\epsilon$]_i [ that [ $\epsilon$_i included high-tech items]]]  (159)0(159

(160)

the export exhibition [ [_NP<<13895>>w$\epsilon$]_i [ $\epsilon$_C [Muriel planned $\epsilon$_i]]] 

The lexicalized nature of XTAG makes it problematic to have trees headed by null strings. Of the two null trees, NP_w and Comp, that we could postulate, the former is definitely more undesirable because it would lead to massive overgeneration, as can be seen in ((161)) and ((162)).

(160)0(160

(161)

* [_NP<<13903>>w$\epsilon$] did John eat the apple? (as a wh-question)  (161)0(161

(162)

* I wonder [[_NP<<13906>>w$\epsilon$] Mary likes John](as an indirect question) 

The presence of an initial headed by a null Comp does not lead to problems of overgeneration because relative clauses are the only environment with a Comp substitution node. ^14.3 Consequently. our treatment of relative clauses has different trees to handle relative clauses with an overt extracted wh-NP and relative clauses with a covert extracted wh-NP. Relative clauses with an overt extracted wh-NP involve substitution of a +<wh> NP into the NP_w node ^14.4 and have a Comp node headed by $\epsilon$_C built in. Relative clauses with a covert extracted wh-NP have a NP_w node headed by $\epsilon$_w built in and involve substitution into the Comp node. The Comp node that is introduced by substitution can be the $\epsilon$_C (null complementizer), that, and for. For example, the tree shown in Figure 14.1(b) is used for the relative clauses shown in sentences ((163))-((164)), while the tree shown in Figure 14.1(a) is used for the relative clauses in sentences ((165))-((168)).

(162)0(162

(163)

the man who Muriel likes  (163)0(163

(164)

the man whose mother Muriel likes 

(164)0(164

(165)

the man Muriel likes  (165)0(165

(166)

the book for Muriel to read  (166)0(166

(167)

the man that Muriel likes  (167)0(167

(168)

the book Muriel is reading 

Cases of PP pied-piping (cf. (169)) are handled in a similar fashion by building in a PP_w node.

(168)0(168

(169)

the demon by whom Muriel was chased 

See the tree in Figure 14.2.

   

Figure: Adjunct relative clause tree with PP-pied-piping in the transitive tree family: $\beta$Npxnx0Vnx1

[height=4.72in]/mnt/linc/xtag/work/doc/tech-rept/ps/rel_clauses-files/NbetaNpxnx0Vnx1.ps