I am sometimes asked what is the secret to the success of our curriculum, what is the special property that sets it apart from other curricula. That question is like the one in the title of this blog post, “Which vertex is the center of a triangle?” It doesn’t have an answer. None of the vertices is the center of a triangle; all three are equally necessary for it to exist. Similarly, all three vertices of the instructional triangle—students, teachers, and content—need equal attention in the work of teaching mathematics. And not only the vertices but the arrows between them, which “represent the dynamic process of interpretation and mutual adjustment that shapes student learning [and] instructional practice.”¹ If there is something special about what we do in writing curriculum it is to pay equal attention to all parts of the instructional triangle.