“…Because students are rarely well versed in disciplinary practices, mathematical practices and conceptual understanding must co-originate (Harel, 2008), in contrast to experts for whom existing well-formed practices inform the development of new disciplinary understandings (Lakatos, 1976;Wilkerson-Jerde & Wilensky, 2011;Wineburg, 1998). To investigate how emerging mathematical practices and concepts could be coconstituted, we conducted a 6-month design study in which middle school students had opportunities to engage in the mathematical practices of definition, conjecture, refutation, inquiry, and proof as they investigated shape and form on the plane (Lehrer, Kobiela, & Weinberg, 2013). In this article, we examine how student participation in the practice of defining influenced their emerging conceptions of the geometry of shape and form and how emerging conceptions of shape and form provided opportunities to exercise and elaborate this form of practice.…”