In this treatise Bartolus applies the Aristotelian schema of constitutions to the city republics of his own day and argues that for the smallest such cities, such as Perugia, where Bartolus lived and worked, the most appropriate form of government is ‘government by the people’ or regimen ad populum. He argues that aristocracy is preferable in larger cities such as Venice and Florence, and then corrects Giles of Rome’s universal endorsement of monarchy as the best constitution by limiting it to much larger political organizations which hold sway over other peoples. He argues that where monarchy is appropriate at all, elective monarchy is superior to hereditary, and established by law for the Roman empire and for the church. Bartolus uses Roman constitutional development as presented in the Roman law to exemplify the different systems of rule brought into being by the growth in numbers and influence of a people. He casts his treatise in part as a lawyer’s version of the Aristotelian constitutional analysis made popular by the theologian Giles of Rome. The concept of the common good is central to Bartolus’s treatment.