Widespread division in Catholic unity caused by rival claims to the Papacy. In the Western Schism (1378-1417) there were two and later three claimants to the Papacy at the same time. The election of Urban VI (1318-89) was challenged post factum by thirteen of his cardinal electors, who in 1378 chose Clement VII as Avignon Pope in his stead. After thirty years of fruitless efforts to settle the rift, a council of prelates at Pisa in 1409 sought to depose the Roman and Avignon pontiffs and elected Alexander V. Finally the schism was healed at the Council of Constance (1414-18). Gregory XII, the Roman Pope, resigned; the antipopes Benedict XII of Avignon and John XXIII of Pisa were deposed, and Martin V (1368-1431) was chosen to replace them.