Emilie Du Chatelet and Christian Wolff both argue, from the principle of sufficient reason, that extended objects and composite objects simpliciter must ultimately be composed of simple beings (monads). Leonhard Euler, who makes extended use of the principle of sufficient reason in his works on mechanics and natural science, argues the contrary: Every extended object is composed of other, composite, extended objects. In this chapter I attempt to locate the differences between these figures that drive them to disparate conclusions. I argue that the difference comes, not from their different applications of the principle of sufficient reason, but from two other sources. First, they endorse different explicability principles which do not themselves follow from the principle of sufficient reasons; and second, they differ with respect to the kinds of explanations (what I call that-explanations and how-explanations) that the question of the composition of extension requires.