The framework of mathematical knowledge for teaching (MKT) is brought under the discursive framework of Commognition in order to track learning in professional development (PD). I follow MKT in differentiating between subject matter discourse and pedagogical discourse. The framework, which I call Mathematical Discourse for Teaching (MDT) permits a combined view on mathematical and meta-mathematical issues as constituted in discourse. Such meta-issues are found to be a significant part of what is taught and learned in a particular PD, where mathematics Ph.D. students teach elementary school teachers. Through the analysis of a lesson on parity I show how "knowing" has different meanings in mathematical and pedagogical discourses, and find evidence of learning in the evolving ways in which the parties use this term. [For the complete proceedings, see ED597799.]