The selection of clausal complement type by embedding predicates constitutes a privileged domain for the assessment of interface issues between different modules of the grammar. This article addresses the selectional problem posed by embedded if-questions (of semantic type <t, t>) appearing as arguments of noninterrogative predicates like 'admit' or 'say' (which are assumed to select a that-clause of type t). We show that such UNSELECTED EMBEDDED QUESTIONS (UEQs) are semantically sensitive to the same set of elements as POLARITY SENSITIVE ITEMS, and this sensitivity constrains their distribution and interpretation. The proposal is that UEQs are headed by a semantically sensitive determiner Δ, which is covert in English (a counterpart of either) but overt in Basque. After raising, the UEQ leaves a variable of type t, thus solving the selectional problem. The interplay between s-selection, c-selection and lexical semantic specifications is argued to account for a number of other puzzles in clausal complementation.