In the early fifteenth century, the general council assembled at Constance and, representing the universal Church, put an end to the scandalous schism which for almost forty years had divided the Latin Church between rival lines of claimants to the papal office. It did so by claiming and exercising an authority superior to that of the pope, an authority by virtue of which it could impose constitutional limits on the exercise of his prerogatives, stand in judgement over him, and if need be, depose him for wrongdoing. In so acting the council gave historic expression to a tradition of conciliarist constitutionalism which long competed for the allegiance of Catholics worldwide with the high papalist monarchical vision that was destined to triumph in 1870 at Vatican I and to become identified with Roman Catholic orthodoxy itself. This book sets out to reconstruct the half-millennial history of that vanquished rival tradition.