this feature is not assigned any value and
is used to co-index moved NPs and their traces which are marked by
possible values are +/-
NPs like who, what etc. come marked from the lexicon with a value of + for the feature wh. Non wh-NPs have - as the value of their wh feature. Note that wh = + NPs are not restricted to occurring in extracted positions, to allow for the correct treatment of echo questions. The wh feature is propagated up by possessives - e.g. the + wh feature of the determiner which in which boy is propagated up to the level of the NP so that the value of the wh feature of the entire NP is +wh. This process is recursive e.g. which boy's mother, which boy's mother's sister. The wh feature is also propagated up PPs. Thus the PP to whom has + as the value of its wh feature. In trees with extracted NPs, the wh feature of the root node S node is equated with the wh feature of the extracted NPs. The wh feature is used to impose subcategorizational constraints. Certain verbs like wonder can only take interrogative complements, other verbs such as know can take both interrogative and non-interrogative complements, and yet other verbs like think can only take non-interrogative complements (cf. the extracted and mode features also play a role in imposing subcategorizational constraints). The wh feature is also used to get the correct inversion patterns.