 Tarski clearly distinguished the object language, i.e., the language the truth of whose sentences is at stake, and the metalanguage in which the former is discussed. He also required (see the entry on  Tarski’s truth definitions)  that any satisfactory definition of truth \(\True(x)\) for the object language should satisfy his “Convention T”, that is, it should have as its consequence all equivalences (“T-equivalences”) of the form