The sentence giving the truth condition of a sentence of an object language, the language under semantic investigation. It is itself framed in a ‘metalanguage’, which is usually in principle distinguished from the object language in order to avoid problems of inconsistency. T-sentences were the lynchpin of Tarski's seminal investigation into how to give a theory of truth for a formal language. A T-sentence takes the form of a biconditional: ‘S’ is true in L if and only if p. Here ‘S’ names a sentence of the object language L, and p is substituted by a sentence in the metalanguage which translates it. Such a sentence is extensional, in the sense that it will be true provided the sentence S has the same truth value as the proposition p. But conditions may be placed on the way the T-sentence is derived in a formal semantic theory, that do something to ensure that the biconditional in fact gives a satisfactory account of the meaning of S.