In this treatise, Bartolus of Sassoferrato explores the phenomenon of factionalism in the fourteenth-century Italian city republics. He gives an account of the local nomenclature of Guelfs and Ghibellines, relating these labels historically to the papal and imperial camps in the contested region of northern Italy. He explains that, nowadays, such labels have only local relevance, not ideological significance. He then analyses the legality of joining such parties, concluding that if the reason for doing so is to uphold the common good, it is lawful.