 Mathematicians have used, and continue to use, diagrams extensively. The communication of mathematical concepts and proofs—in textbooks, on blackboards—is not uniformly sentential. Figures and pictures are common. In line with the prevailing conception of logic as essentially sentential, however, they are not usually thought to play a role in rigorous mathematical reasoning. Their use is taken to be limited to enhancing comprehension of a proof. They are not standardly believed to form any part of the proof itself. 