Communication is an integral part of classroom interaction and the quality of communication influences the quality of teaching and learning mathematics. In general, the communication is about a content, and how the content is handled during the communication has an impact of what aspects the students have the possibility to discern. The communication in the classroom succeed or not depending on the opportunities offered in the classroom to work out the meaning of the whole by knowing the meaning of the simple parts, the semantic significance of a finite number of syntactic modes of composition, and recognizes how the whole is built up out of simple parts. In the presentation the focus on a the effectiveness of communication in mathematics classrooms, and it is based on data collected, during a 3-year period, while different objects of learning is presented in nine classes, and it includes 22 teachers and 884 students.