An interrogative clause is a clause p of a clause type characterized by the basic illocution ‘interrogative’. This comprises the following properties of p:

1. P presupposes a disjunctive configuration of propositions which are possible specifications of p.
2. The author of p does not choose among these alternatives, thus explicitly leaving the disjunction unresolved.

There are two types of interrogative clauses; they differ in the paradigmatic relationship between the presupposed disjunct propositions:

1. A polar interrogative presupposes ‘p or not p’ (p ∨ ¬p).
2. A content interrogative is an open proposition containing a variable x. It presupposes ‘x is x1 or xi ... or xn’ (x ϵ x1 ∨ x ϵ xi ... ∨ x ϵ xn).

a illustrates a polar interrogative; #b illustrates a content interrogative.

A polar interrogative may be marked by a formative indicating the binary disjunction, like whether in a. A content interrogative is generally marked by an indefinite (or specifically interrogative) proform representing the variable x, like where in the #b examples.

Just as clauses of the other clause types, an interrogative clause may be direct (independent) or indirect (subordinate).

• A direct interrogative clause is an interrogative sentence. It may be used as a question.
• An indirect interrogative clause is a substantive clause, typically dependent on a predicate coding a propositional attitude (like ‘ask’) in which the actor (the author) leaves the above disjunction unresolved.

illustrates direct interrogatives; illustrates indirect interrogatives.

If an interrogative sentence is used as a question, it expects an answer and therefore passes the turn to the interlocutor.

• A polar interrogative sentence expects a yes/no answer.
• A content interrogative sentence expects information replacing the interrogative proform.

Alternative terms for the two types of interrogatives have been in use:

1. Polar interrogatives have been called “yes/no” interrogatives.
2. Content interrogatives have been called “question-word interrogatives”, “pronominal interrogatives” or “wh-interrogatives”. Of these, ‘pronominal interrogative’ is too narrow, since the proform meant does not need to be a pronoun (it is not in the examples). ‘Wh-interrogative’ is anglocentric (and does not even cover how interrogatives).

Moreover, in all of these alternative terms, the word interrogative is not seldom replaced by the word question. However, the present topic is a clause type, not an illocutionary act. Indirect interrogatives are not normally used as questions. Even direct interrogatives are often not so used, e.g. in a rhetorical question.