 We conclude with some remarks on a question that was briefly mentioned above in connection with the Special Composition Question but that pertains more generally to the underlying notion of parthood that mereology seeks to systematize. All the theories examined so far, from M to GEM and its variants, appear to assume that parthood is a perfectly determinate relation: given any two entities x and y, there is always an objective, determinate fact of the matter as to whether or not x is part of y. However, in some cases this seems problematic. Perhaps there is no room for indeterminacy in the idealized mereology of space and time as such; but when it comes to the mereology of ordinary spatio-temporal particulars (for instance) the picture looks different. Think of objects such as clouds, forests, heaps of sand. What exactly are their constitutive parts? What are the mereological boundaries of a desert, a river, a mountain? Some stuff is positively part of Mount Everest and some stuff is positively not part of it, but there is borderline stuff whose mereological relationship to Everest seems indeterminate. Even living organisms may, on closer look, give rise to indeterminacy issues. Surely Tibbles's body comprises his tail and surely it does not comprise Pluto's. But what about the whisker that is coming loose? It used to be a firm part of Tibbles and soon it will drop off for good, yet meanwhile its mereological relation to the cat is dubious. And what goes for material bodies goes for everything. What are the mereological boundaries of a neighborhood, a college, a social organization? What about the boundaries of events such as promenades, concerts, wars? What about the extensions of such ordinary concepts as baldness, wisdom, personhood? 