Mathematics teachers make a number of decisions that shape their lessons, which therein impact their students' opportunity to learn mathematics. Past research has often focused on teachers, students and the mathematical content as key classroom elements that drive teachers' decisions. In this article, we propose that a fourth element -- the curriculum -- along with teachers' curricular reasoning also hold considerable influence on teachers' decisions. Using data collected from middle grades teachers' curricular decisions, we share the Instructional Pyramid model for Curricular Reasoning to organize the interactions among these four key classroom elements and to delineate aspects of curricular reasoning. This model serves as a multi-dimensional framework to make sense of teachers' curricular decisions.