Amid one of the most turbulent periods in the history of the medieval papacy, Gregory X: The Pope Who Changed the Rules of Power reconstructs, with historical rigor and narrative clarity, the life and legacy of Teobaldo Visconti, an unexpected occupant of the throne of Saint Peter. His election after the longest papal election in history (nearly three years of internal divisions, enforced confinement, and external pressure) marked a turning point in thirteenth-century ecclesiastical politics. From his beginnings as a cleric from Piacenza, archdeacon of Liège, and crusader in the Holy Land to his decisive role at the Second Council of Lyon, the pontificate of Gregory X (1271–1276) was brief yet profoundly reformist. This book examines his attempt at reconciliation with the Greek Orthodox Church, his promotion of the institutionalization of the conclave as the system for papal election, and his efforts to establish an equilibrium between the European powers and Rome’s spiritual authority. Combining political, ecclesiological, and diplomatic analysis, the work offers a portrait of a pontiff whose historiographical reception has been uneven and, above all, a study of a government that left enduring institutional instruments to the medieval Church. This series brings together independent studies that explore power, ideology, legitimacy, and history from a cross-disciplinary perspective. Each volume is an autonomous work, yet all form part of a common thread: the critical analysis of how power is configured, exercised, and sustained over time. From ancient Egypt to twentieth-century totalitarianism, Working Papers presents, with academic rigor and clear prose, the historical and discursive mechanisms that shape our societies. Each installment is numbered to indicate its place within this evolving series.