This is so because there can be found structures where it seems as if the C-head position is not filled with anything.
The presence of Tense in the C-head being overtly supported by the insertion of the verb do is a good indicator that what indeed occupies the specifier of the CP and not C itself.
In the case of an embedded wh-CP, when the internally merged expression is what, the system is equally directed towards two choices: either the wh-expression projects as the CP-label, thus rendering a free-relative interpretation, or the C-Head itself projects, thus yielding a wh-interrogative interpretation.
Let's assume that in the context of clause-embedding and in accord either with general locality considerations (Rizzi 2004) or some constraint adhering to impenetrability (Chomsky 2001) of the CP domain to anything that lies above the embedded C-head, it is not the verb itself that attracts the wh-expression but C derivatively, due to it being marked as interrogative:
Rizzi's analysis crucially does not account for the underlying reasons of the asymmetry between matrix and embedded clauses that render the former defective regarding the C-head's [+wh]- specification.
The C-head's selectional properties are satisied in the structural context of sisterhood and Tense tampers with its featural constituency overtly marking it for Tense.
This Roller C-Head
ran on Reifenhauser's three-layer Filmtec demonstration line with an automatic die.
There is then a one-way dependency between the phonological and lexically defined semantic structuring that surfaces as the ability of an item to move into the C-head position in syntax.
The reason then that which is excluded from occurring in the C-Head position although it can stand overtly on its own is that its ability to repair its maximality is not only a sufficient condition for its grammatical interpretation but also a necessary one.
In contrast, the lexically determined context-independence of what makes it eligible for occupying C-head position, a choice that does not clash with the denotational properties of what.
The wh-Restrictor in the case of free relatives comprises exclusively what the complement of the C-head compositionally denotes.
This is the very reason that wh-head movement blocks d-linking, thus disallowing which from occupying C-head position.
The Roller C-Head
sensor comes from Plast-Control, which recently moved to offices in Newburyport, Mass.
Pro-Con consists of a closed-loop computer control, a newly designed contact capacitance gauge called the C-head, and a new segmented air ring.
The C-head and computer for profile measurement cost $25,000-$55,000.