 Federations are often thought to be sui generis, one-of-a-kind deviations from the ideal-type unitary sovereign state familiar from the Westphalian world order. Indeed, every federation may well be federal in its very own way, and not easy to summarize and assess as an ideal-type political order. Yet the phenomenon of non-unitary sovereignty is not new, and federal accommodation of differences may well be better than the alternatives. When and why this is so has long been the subject of philosophical, theoretical and normative analysis and reflection. Such public arguments may themselves contribute to develop the overarching loyalty required among citizens of stable, legitimate federations, who must understand themselves as members of two commonwealths.