This paper presents a reading of Metaphysics Z.10–11 according to which both chapters outline two main definienda: forms and material substances or compounds, each of which is governed by its own peculiar constraints. Forms include formal parts alone; furthermore, they are the main definable items and enjoy the strictest possible unity. However, this does not preclude Aristotle from upgrading material compounds to the status of definable items in their own right. Z.10 explains this contention by making the compound’s sensible functional (...) matter dependent on its form. This dependence affords the resulting compound an unusually tight form-matter relationship, strong enough to ensure its definability, despite falling short of the highest kind of unity that only forms display. (shrink)