In this paper, we analyze mathematics lessons using the Classroom Assessment Scoring System (CLASS), a standardized observation protocol that suggests that high-quality lessons are distinguished by the tenor and frequency of classroom interactions. Because the CLASS focuses on interactions, rather than the specifics of content teaching, it can be used across content areas from language arts to mathematics. While many previous studies have used CLASS as a measure of instructional quality, to date, no work has examined the affordances and constraints of using the content-agnostic CLASS to examine instructional quality in mathematics lessons. Our close qualitative analysis of three lessons highlights the importance of including practices that cut across content areas in measurement of instructional quality in mathematics classrooms. In addition, this paper is the first to highlight aspects of instruction in mathematics classrooms that are obscured by the CLASS. Discussion highlights how a relational lens foregrounds particular instructional aspects and marginalizes others.