It is controversial whether masses (what mass nouns refer to) exist. But on the assumption that they do, here are two uncontroversial facts about them: first, they satisfy a fusion principle which takes any set of masses of kind K and yields a mass fusion of kind K; secondly, a mass must have all and only the same parts at every time at which it exists. These two theses are usually built into the concept ‘mass’. I argue that the latter follows from the former. This shows that the concept ‘mass’ is unified, not gerrymandered. Moreover, since my arguments show that any entity which follows a certain fusion principle is also mereologically constant, and since these two properties are sufficient for being a mass, my arguments make it easier to argue that there are masses.