Limited literature addresses how elementary preservice teachers (PSTs) can advance their thinking about hierarchical geometric relationships. Informed by a commognitive perspective, we investigated this phenomenon as a matter of discursive change, focusing on word use, visual mediators, and narratives. We report on a teaching experiment involving a PST whom we call Mariah. Mariah's progress seemed to be driven primarily by metalevel changes in her word use and interpretations of diagrams. These changes supported shifts in her narratives regarding hierarchical geometric relationships. Instructor moves related to these discursive changes involved brokering between multiple discourse communities. Our analysis reveals nuances of communication that we do not see highlighted in the literature on PST education. [For the complete proceedings, see ED657822.]