thatthat

Thatthat is a descriptive label used in English linguistics to refer to a sequence in which the word that appears twice in immediate succession within a single sentence. In this pattern, the first that typically functions as a complementizer introducing a that-clause or as a demonstrative determiner, while the second that acts as a demonstrative pronoun referring to a proposition or to a prior noun phrase. The surface string can be ambiguous without context or punctuation.

Examples illustrate the pattern. I know that that is true. Here the outer that introduces the clause, and the inner that functions as a pronoun referring to the proposition that is true. The claim that that is false is ungrounded. In this case, the first that begins the embedded clause, while the second that stands in for the proposition “that is false.” That that idea is accepted surprised the team shows a similar structure, with the initial demonstrative forming a referent for what follows.

Because of potential ambiguity, writers often prefer rewritten forms or added punctuation to reduce processing difficulty. For instance, I know that that is true can be rewritten as I know that proposition is true, or I know that the statement is true, depending on context. The phenomenon is discussed in studies of syntactic ambiguity, pronoun reference, and pro-forms, illustrating how English can recycle the word that in different grammatical roles within the same sentence.