The production of mathematical formalism on state of the art computers is quite different than by pen and paper. In this paper I examine the question of how different media influence the writing of mathematical signs. The examination is based on an investigation of professional mathematicians' use of various media for their writing. A model for describing mathematical writing through turntakings is proposed. The model is applied to the ways mathematicians use computers for writing, and especially it is used to understand how interaction with the computer system LaTeX is different in the case of formulas than in the case of diagrams.