This paper presents an analytic approach for understanding the interplay through time between “scientific” and “everyday concepts” in a mathematics classroom community. To illustrate the approach, we focus on an elementary classroom implementing an integers and fractions lesson sequence that makes use of the number line as a principal representational context. In our analysis of the community's emerging collective practices (recurring structures of joint activity), we trace the interplay between children's sensorimotor actions (displacing, counting, and splitting) and the mathematical definitions supported in the classroom, like definitions of unit interval or equivalent fractions. In our illustrative analysis, we find that the teacher orchestrated collective practices to support the use of actions to make sense of the formal definitions, and the use of definitions to regulate actions. Though we illustrate the analytic approach for a particular classroom community, the approach illuminates teaching-learning dynamics that transcend any particular classroom or subject matter domain.