This article discusses how communication theory is used to understand the thoughts mathematics teachers employ when creating messages intended for students. According to communication theory, individuals have different premises about the act of communicating, and these thoughts, called message design logics, guide the process of reasoning from goals or intentions to actual messages (O’Keefe, 1988, 1990). Three distinct message design logics have been identified by communication theorists: expressive, conventional, and rhetorical. Depending upon which logic an individual employs, a very different message is said and heard. This theory was used to investigate the message design logics of 15 secondary mathematics teachers. It was found that teachers have varying logics in their message production and, depending upon the logic used, distinct characteristics correspond to different teacher premises for classroom communication.. The logic employed also results in different ways teachers encourage mathematical learning and evaluate classroom interactions.