One of the hallmarks of w h-movement is that wh-phrases in clause-initial position are associated with a more deeply embedded interpretative gap, as indicated by the structures in (507a&b). Such structures can be used as input for the semantic component of the grammar and be translated into semantic representations with a question operator and a variable, as in the primed examples.

There are several conditions on operator-variable representations in natural language that are not assumed for their counterparts in formal-logical systems. For example, while formal-logical systems allow vacuous quantifiers, that is, quantifiers that do not bind a variable, natural language does not. This can be seen as the result of a more general economy condition on natural language which prohibits superfluous elements in a representation: sentence (508a) is unacceptable despite the fact that a semanticist may consider its formal semantic counterpart in (508b) impeccable; cf. Chierchia & McConell-Ginet (1992:110).

Since a variable must be bound by an operator in order to form an interpretable sentence, the fact that the examples in (509) are uninterpretable does not come as a surprise; we will ignore the fact here that we do find constructions like (509a) in certain (e.g., generic) contexts that allow an implied theme argument and with pseudo-intransitive verbs, that is, verbs that take a cognate object.

W h-moved phrases further differ from semantic operators in that they can normally bind a single interpretative gap at most: a sentence like $Wie heeft onderzocht?'Who has examined?' cannot be assigned the meaning indicated by the well-formed semantic representation in (510b); the only way to express this meaning is by using a reflexive pronoun: Wie heeft zichzelf onderzocht?'Who has examined himself?'.

Koopman & Sportiche (1982) account for the observations above by postulating that natural language is subject to the bijection principle in (511); the specific phrasing of the principle is taken from Webelhuth (1992:143).

This subsection discusses an (apparent) problem for clause (511a) of the bijection principle in the sense that a single wh-phrase is related to more than one interpretative gap. Such cases have been studied intensively for English since Engdahl's (1983) seminal paper on this issue, but has received less attention in other languages. We will therefore introduce the notion of parasitic gap gap by using English examples. The results can then be used as a starting point for our description of Dutch in Subsection III. The discussion below is based on the more extensive review found in Culicover (2001).
      A standard example of a parasitic gap construction from English is (512a); given that the two interpretative object gaps are translated as variables bound by the same question operator in the informal semantic representation in (512b), this example seems to violate clause (511a) of the bijection principle.

The use of a trace in the object position of the main clause in (512a) is motivated by the fact that it can be independently established that wh-movement is possible from this position; cf. Which articlesi did John file ti? The reason for usingthe notion parasitic gap (pg) for the interpretative gap in the adverbial phrase is twofold. First, example (513a) shows that it cannot be a trace left by wh-movement of who, as adverbial clauses are islands for wh-extraction. Second, example (513b) shows that it cannot occur if the direct object of the main clause occurs in its base-position; the gap is thus "parasitic" on wh-movement of this phrase.

Parasitic gap constructions are not limited to wh-questions but also occur in other constructions derived by wh-movement. This is illustrated in example (514a) for a relative clause; examples (514b&c) show that wh-movement of the phonetically empty relative pronoun OP is possible from the object position of the relative clause but not from the object position of the adverbial clause. Note in passing that Engdahl assigns (514c) a question mark, while we use an asterisk: this is because Culicover (2001) simply calls this example ungrammatical.

Culicover (2001) provides a number of properties of parasitic gap constructions that are generally accepted, while noting that these claims have all been challenged in the literature at some point. An adapted version of his list is given as (515).

Landing-site restriction (515a) refers to the fact that parasitic gap constructions typically occur in constructions derived by wh-movement; the English examples given above illustrate this point. This has led to the claim that the antecedent of the trace and the parasitic gap cannot be in an A-positions (that is, argument positions to which thematic roles, agreement features and/or case are assigned) but must be in an A'-position, which may account for the fact that parasitic gaps may also occur in, e.g., English heavy NP-shift constructions. We will see, however, that this claim is not generally accepted for Dutch parasitic gap constructions.
      The overt-movement restriction in (515b) is based on the standard generative assumption from the 1980's that wh-elements in situ undergo covert movement, that is, movement after the structure has been transferred to the phonological component of the grammar. Although this claim is no longer accepted by many generative linguists, the empirical issue still remains, which is that parasitic gaps cannot be licensed by wh-phrases occupying their base position; the wh-phrase which article in multiple question (516) does not license a parasitic gap. For convenience we will maintain the notion of overt-movement restriction without implying a specific stance on the issue of covert movement.

The anti-c-command restriction in (515c) on the relation between the wh-trace and the parasitic gap can be derived from binding condition C, which forbids referential expressions to be A-bound, that is, to take a c-commanding antecedent in an argument position. This is done by extending to parasitic gaps the standard claim that wh-traces of nominal arguments exhibit the same binding behavior as referential expressions. The anti-c-command restriction can be used to account for the fact that subject traces block parasitic gaps more deeply embedded in their own clause, as illustrated by (517a&b), while traces left by wh-extraction of a subject from an embedded clause do not block parasitic gaps in matrix clauses, as illustrated by (517c). The examples are taken from Engdahl (1983) and Chomsky (1986); we will discuss a problem for the claim that wh-traces and parasitic gaps are subject to binding condition C in Subsection IIID, but we will accept this claim as a working hypothesis for what follows.

The anti-c-command restriction also predicts the acceptability of examples like (518a&b), which are again taken from Engdahl (1983) and Chomsky (1986). It also accounts for the fact illustrated in the primed examples that substituting a simple gap for the complex noun phrase a picture of pg i is impossible: because the two gaps are both A'-bound by the wh-phrase in clause-initial position and the first gap c-commands the second, the second gap is incorrectly A-bound by the first gap. Note that on the assumption that nominal wh-traces and parasitic gaps are both subject to binding condition C, this result follows regardless of whether the first or the second gap is considered to be parasitic on wh-movement; we therefore did not specify the nature of the gaps in the primed examples.

It should be noted that we can only maintain the anti-c-command restriction if we assume that the direct objects in (512)-(514) do not c-command the adjuncts containing the parasitic gaps. This assumption is consistent with the fact that complements are generated as the immediate sister of the selecting verb, but inconsistent with the c-command hierarchy that we introduced in Section N5.2.1.5, sub III; we refer the reader to the discussion of this issue in Contreras (1984), Koster (1987: Section 6.4) and Safir (1987), and to Lasnik (1999:ch.6) for a specific approach to English objects that may solve this problem.
      Categorial restriction (515d), according to which the wh-moved phrase must be nominal, has been claimed not to be cross-linguistically valid but can at least be seen as a strong tendency in English: wh-movement of APs or PPs normally does not license parasitic gaps. Two examples adapted from Cinque (1990:115) are given in (519); see Koster (1987:156-7) for more examples.

The examples above have shown that parasitic gaps are typically found in islands for wh-extraction, such as the adjuncts in (512) and (514) or the subject in (518b). Kayne (1984:ch.8) and Contreras (1984) have noted, however, that parasitic gaps cannot be embedded in islands within an island, as stated by the multiple-island restriction in (515e). This is illustrated by the contrasts in acceptability indicated in (520) and (521), in which the two (b)-examples should be construed as alternative realizations of the adverbial clauses in the (a)-examples, and the abbreviation OP again indicates the phonetically empty relative pronoun.

      Kayne detects a "sharp contrast" between the two alternative realizations of the adjunct clauses and attributes this to the fact that the parasitic gaps are embedded in a single (adjunct) island in the primeless (b)-examples but in two islands in the primed examples, an adjunct island and an additional subject island.
      Now that we have briefly discussed the five restrictions in (515), we conclude our brief survey of English parasitic gaps by noting that Engdahl (1983) has found a great deal of variation in speakers' judgments on parasitic gap constructions. Furthermore, it seems that the acceptability of parasitic gap constructions depends on the phrases they are embedded in; parasitic gaps in non-finite clauses such as (520b) are more likely to be accepted by speakers than parasitic gaps in finite clauses such as (521b). Or, stated somewhat differently, speakers who accept parasitic gaps in finite adjunct clauses such as (521b) will also accept them in non-finite adjunct clauses such as (520b), while the inverse does not necessarily hold. Engdahl's hierarchy is given in a shorter and slightly adapted form as (522): it expresses that parasitic gaps are best in infinitival adjunct clauses, somewhat less favored in finite argument/adjunct clauses, and least favored in relative clauses.