This article offers a particular analytic method from systemic functional linguistics, "thematic analysis," which reveals the mathematical meaning potentials construed in discourse. Addressing concerns that discourse analysis is too often content-free, thematic analysis provides a way to represent semantic structures of mathematical content, allows for content comparisons to be drawn between classroom episodes, and identifies points of possible student misinterpretation. Analyses of 2 middle school classroom excerpts focusing on area--1 that derives triangle area formulas from the rectangle area formula and another that connects parallelogram and rectangular area--are used to delineate the method. Descriptions of similarities and differences in the classroom discourse highlight how, in each classroom, mathematical terms such as "base" and "height" were used in semantically related but distinct ways. These findings raise the question of whether students were aware of and able to navigate such semantic shifts. (Contains 9 figures, 3 tables and 15 footnotes.)