This article reviews the literature base on class size reduction and proposes a new analytic framework that we believe provides practically useful explanations of how primary class size reduction works. It presents descriptions of classroom practice and grounded explanations for how class size reduction affects educational core activities by considering the influences of policy space, physical space, and spatiality. By reporting on Ontario, Canada's Primary Class Size Reduction (PCS) Initiative, this article illuminates the factors that may occur in tandem with class size reduction--treating it not a simple policy change, but rather as a cluster of interrelated dynamics. (Contains 2 figures.)