So far, we have implicitly assumed that CP comprises a head C constituent (which can be filled by a complementizer or a preposed auxiliary) and a TP complement. However, one question which such an analysis begs is what position is occupied by the bold-printed constituent which precedes the italicized auxiliary in root interrogatives (i.e. main-clause questions) such as (1) below:

Each of the sentences in (1) contains an italicized inverted auxiliary occupying the head C position of CP, preceded by a bold-printed interrogative wh-expression – i.e. an expression containing an interrogative word beginning with wh- like what/which/who/where/when/why. (Note that how in questions like How are you? How well did he behave? etc. is also treated as a wh-word because it exhibits the same syntactic behavior as interrogative words beginning with wh-.) Each of the wh-expressions in (1) functions as the complement of the verb at the end of the sentence – as we see from the fact that each of the examples in (1) has a paraphrase in which the wh-expression occupies complement position after the italicized verb:

Structures like (2) are termed wh-in-situ questions, since the bold-printed wh-expression does not get preposed, but rather remains in situ (i.e. ‘in place’) in the canonical position associated with its grammatical function (e.g. what languages in (2a) is the direct object complement of speak, and complements are normally positioned after their verbs, so what languages is positioned after the verb speak). In English, wh-in-situ questions are used primarily as echo questions, to echo and question something previously said by someone else – as we can illustrate in terms of the following dialogue:

Echo questions such as that produced by speaker B in (3) suggest that the wh-expressions in (1) originate as complements of the relevant verbs, and subsequently get moved to the front of the overall clause. But what position do they get moved into?

The answer is obviously that they are moved into some position preceding the inverted auxiliary. Since inverted auxiliaries occupy the head C position of CP, let’s suppose that preposed wh-expressions are moved into a position preceding the head C of CP. Given that specifiers are positioned before heads, a plausible suggestion to make is that preposed wh-expressions move into the specifier position within CP (= spec-CP). If so, a sentence like (1c) Who was she dating? will involve the arrowed movement operations shown in (4) below:

(To be more precise, interrogative pronouns like who are Q-pronouns and hence pronominal quantifiers.) Two different kinds of movement operation (indicated by the numbered arrows) are involved in (4): the movement arrowed in (1) involves the familiar operation of head movement by which the bold-printed auxiliary was moves from the head T position of TP into the head C position of CP; by contrast, (2) involves movement of an italicized wh-expression from the complement position within VP into the specifier position in CP, and this very different kind of movement operation is known as wh-movement. Note that unlike head movement (which, as its name suggests, moves only heads which are minimal projections), wh-movement moves maximal projections; for instance, in (1a) What languages can you speak? wh-movement moves the quantifier phrase what languages which is the maximal projection of the interrogative quantifier what? by virtue of being the largest expression headed by the word what; and in (1c) Who was she dating? it moves the interrogative Q-pronoun who (which is a maximal projection by virtue of being the largest expression headed by the word who). Following Cheng (1997), we might suppose that every clause must be typed (i.e. identified as declarative or interrogative etc. in type) in the syntax, and that a clause is typed as interrogative if it contains an interrogative head or specifier: on this view, movement of the interrogative pronoun who to spec-CP serves to type the CP in (4) as interrogative.

Evidence in support of the assumption that preposed wh-expressions move into spec-CP comes from varieties of English in which a preposed wh-expression can precede a complementizer like that. This is true, for example, of interrogative complement clauses like those bracketed below in Belfast English (from Henry 1995, p. 107):

Since the complementizer that occupies the head C position in the bracketed CP, it seems reasonable to suppose that the wh-expressions which dish/which model in front of that occupy the specifier position within CP, and this is what Alison Henry argues. (See Sepp¨anen and Trotta 2000 and Zwicky 2002 for discussion of the syntax of wh+ that structures.)