From the fourteenth to the eighteenth century the union of Poland and Lithuania, formally a commonwealth as of 1569, included Poles and Lithuanians, Ruthenians, Prussians, Galicians, German immigrants, Cossacks, and Jews, overlaid by religious patchwork of Catholics, Eastern Orthodox Christians, Uniates, Lutherans, Calvinists, Kariates, and pagans. In its heyday it stretched from the Baltic to the Black Sea, making it the largest polity contained entirely in Europe. How did racial, ethnic, and religious identities of these constituent “nations” (gentes, narody) function in political membership under the “umbrella” (Friedrich) of the Polish-Lithuanian state? How did their polity compare to other Early Modern “composite” states (Koenigsberger, Butterwick), for example Spain? How did forces of cohesion and fragmentation play out over time, and are there any lessons here to inform our understanding of Europe in subsequent centuries, into the present?