The Liber Pontificalis, the official collection of papal biographies, as well as other Church histories, provides considerable information on the development of election law. As this summary shows, it was often developed in the crucible of trial, as the Church sought to fend off, or at least moderate, external influences on the election process.
Cletus (76-88) ordains 25 priests to serve Rome, the earliest extra-biblical ordination known.
Evaristus divided Roman titles among the priests and ordained 7 deacons to assist him.
Fabian divided the Roman See into 7 deaconal regions. The deacons who administered these regions, together with the 7 deacons who were the assistants to the Pope, were the forerunners in office of the Cardinals who have diaconal rank today.
Pope Mark establishes the Bishop of Ostia as the principal consecrator of the newly elected Pope. At this time in history the popes typically came from among the priests and deacons of Rome, and thus had to be consecrated a bishop in order to assume office.
Boniface I (418-22) obtains the agreement of Emperor Honorius that in a disputed election (such as his was), both electees would be rejected. The civil authorities would be obliged to recognize a unanimous electee. The interference of Emperor Honorius in the election of Pope Boniface was the first in Church history, but not the last. Likewise, this agreement would not be the last attempt to ensure the freedom of the election and the tranquility of the passing of papal authority.
Symmachus (498-514) calls a Roman Synod which decrees as he wished that only the clergy of Rome will vote in an election and the majority candidate wins. Such election would occur if the previous Pope did not designate his successor.
Boniface III (607) calls a Roman Synod which reinforces the prohibition of electioneering and bribery. It also establishes a 3 day wait for the election after the death of a Pope. It also permitted the participation of the nobility, along with the clergy.
After Gregory III (731-41) Popes no longer though it necessary to seek the concurrence of the Emperor in Constantinople. While no Pope had thought it a theological necessity it had become a practical one for election. However, it was one under which the Bishop of Rome chaffed. Often the Emperors had their pet heresies which Rome resisted as heterodox, and the process of seeking his consent, or that of his representative in Ravenna, often delayed the inauguration of the pontificate by weeks or even months. Eventually, the emperor's problems in the East with Islam effectively liberated the Pope from the need for his approval.
Stephen III (IV) sought to restrict the electors of the Pope. Acting through a Roman Synod it was decreed that only cardinal priests and cardinal deacons could be papal electors. At this time in history the cardinals actually held the offices which today are only titular. Thus, the priests and deacons in question were the actual pastors of principal Roman churches and deacons who administered Roman districts. The role of the laity and nobility was restricted to confirming the election.
Leo III crowns King Charles (Charlemagne) of the Franks, Holy Roman Emperor, re-establishing the Western empire.
The Co-Emperor Lothair insisted on the rights of all Romans, clerical and lay, to participate in papal elections. The emperor would not interfere in the election, but would require an oath of allegiance to himself of the electee.
Nicholas I, acting through a Roman Synod, condemned those who denied the election of the Bishop of Rome by its clergy and nobility alone.
John IX at the Synod of Rome decrees that in the future the Pope will be elected by the cardinal bishops, cardinal priests and cardinal deacons. The rights of the nobility and people were protected by requiring the election to take place publicly.
John XII agrees to the Ottonian Privilege, which required the Pope to take an oath of loyalty to the Holy Roman Emperor, Otto at that time, in exchange for his protection and non-interference in papal elections.
Nicholas II, in the synodal decree In nomine domine establishes that only cardinal bishops will elect the pope, and then, as a usual matter, from among the Roman clergy. The cardinal priests and deacons would then be asked to give their consent. Finally, consent would be sought from the people. The emperor could confirm the Pope, but this was a privilege, not a right.
Gregory VII (Hildebrand) began the fight against the practice of investiture, secular princes "investing" popes and bishops with office. In this he ran head-long into Henry IV, whom he excommunicated and unsuccessfully deposed.
Urban II grants equal rights to cardinal priests, on par with cardinal bishops, in the election of popes.
Finally, in the Concordat of Worms the Emperor agreed that secular princes did not convey spiritual authority (symbolized by the episcopal crosier and ring) to the bishops in whose territory their diocese falls. However, they could convey the scepter as a sign of any temporal rule that fell under the bishops jurisdiction.
Eugene III gives the Sacred College of Cardinals canonical form, with a Dean, who is Bishop of Ostia, and a Camerlengo, who is administrator of property.
Alexander III allows the Archbishop of Mainz to return to his See in Germany rather than remain in Rome, as Cardinals were required to do. He gave him a "titular" church in Rome, thus instituting the practice which is today the norm.
The 3rd Ecumenical Council of the Lateran, convened by Pope Alexander III, decrees that the one elected must have 2/3rds of the votes of the Cardinals who are present.
The red hat seems to have been first granted to Cardinals under Innocent IV.
The Constitution Ubi periculum of the 3rd Ecumenical Council of Lyon, under Gregory X, establishes norms for the death and election of a Pope. It decrees the cloistering of the Cardinals and the secrecy of the Conclave behind a double lock. The Marshal of the Conclave holds the outside key, the Camerlengo the inside key. If an election does not result in three days, the Cardinals are to be have only bread, water and wine.
Hadrian IV suspends the harsh norms of Ubi periculum.
Celestine V in Constitutionem revises and renews the norms of Ubi periculum. He provides three means of election: 1) 2/3rds vote in a secret ballot, 2) acclamation (also called inspiration), whereby one or more Cardinals propose someone whom the rest accept, or 3) consensus, in which election is achieved by mediation between blocks of votes.
Boniface VIII confirms the legislation of Celestine V and Gregory X inserting it into the canon law decretals. He also grants the wearing of a red cassock to Cardinals.
Innocent VI condemns and nullifies certain concessions regarding the authority of the Cardinals agreed to during the conclave which elected him, thus asserting the invalidity of such bargains which attempt to restrict the authority of the Pope.
Martin V seems to have created the first Cardinal "in pectore", that is, without publicly naming him. Such cardinals loose their title if not "published" prior to the Pope's death.
Eugene IV establishes that Cardinals do not receive their rank and title until the received their insignia of office.
Red biretta, red zucchetto (skull-cap), and red mantle, began to be bestowed on new Cardinals under Paul II.
Paul IV establishes that the most senior Cardinal residing in Rome becomes the Dean of the College.
Council of Trent, under Pius IV, decrees that the Cardinals should be chosen from all nations in Christendom and if they are residential bishops they should remain in their respective Sees rather than live in Rome.
Sixtus V, in his apostolic constitution Postquam verus decrees that Cardinals be clerics for at least one year prior to creation, which meant they had received all minor orders (porter, lector, exorcist, acolyte). Further, they could not be illegitimate sons, have any impediments for receiving holy orders, have had children or grandchildren, nor be the brothers, nephews, uncles, cousins, or any relative of first or second degree, of members of the Sacred College.
Sixtus V grants quasi-episcopal jurisdiction to the Cardinals who hold Roman titles and deaneries.
Urban VIII granted the title Eminence to Cardinals.
Innocent XII, in his apostolic constitution Romanum decet Pontificem transfers jurisdiction over the clergy and people of the Roman churches and deaconries from the Cardinals who hold title to the Cardinal Vicar of Rome. The Cardinals retained some rights and privileges, however.
Innocent XII, in his apostolic constitution Ecclesiae Catholicæ forbade the practice of "capitulations", agreements between the cardinals and a papabile about what he will do after he is elected.
Benedict XIII decreed that the Cardinal Dean will be the most senior Cardinal, irrespective of residence.
Clement XII restored the Deanship to the most senior Cardinal residing in Rome. He also provided that precedence within the 3 ranks of Cardinals be established by seniority, seniority of episcopal consecration for Cardinal Bishops, and seniority of being made Cardinal for the other ranks.
Leo XIII provides for holding a papal election in extraordinary circumstances, such as the occupation of Rome (begun in 1870 by Italian forces), through his Apostolic Constitution Praedecessores Nostri.
Pius X establishes an excommunication for any Cardinal who would attempt to exercise a "right of exclusion" (veto) on behalf of a secular prince (at this time Austria, France and Spain). He also establishes comprehensive laws regarding the death of a Pope, the interregnum and the election of the Pope, in the Apostolic Constitution Vacante Sede Apostolica. Among these, the Cardinals must wait ten days before beginning the election.
Pius X establishes suffragan bishops in the suburicarian Sees to relieve the Cardinal Bishops of the responsibility for daily administration.
Pius X divides the See of Velletri and Ostia and decrees that the Dean of the College will hold the See he held at the time of election as Dean, as well as the See of Ostia.
Benedict XV restores the full governance of the Cardinal Bishops over the suburicarian Sees.
Benedict XV promulgates the 1917 Code of Canon Law, which consolidates the general law of the Church. May other laws remain separate, such as those regarding the organization of the Roman Curia and the Death and Election of a Pope.
Pius XI establishes that the Conclave must begin fifteen days after the death of a Pope and no later than eighteen.
Pius XI signs the Lateran Treaty with Italy. Among its provisions is the freedom of the conclave.
Pius XII, in his apostolic constitution Vacantis Apostolicæ Sedis repeats most of the norms promulgated by Pope Pius X. However, he establishes a 2/3rd + 1 majority for election, changing the norm of Lateran III (1179).
Cardinals Bishops will have title to their Sees in name only. Other bishops will have full ordinary power in those dioceses.
Paul VI decrees in Ad purpuratorum patrum, issued motu proprio, that Eastern-Rite Patriarchs made Cardinals will not be given Roman title, but will retain their patriarch Sees.
Paul VI's Ad hoc usque tempus, issued motu proprio, abolishes all administrative and governance functions of Cardinals priests and deacons in the churches and deaconries to which they hold title. They retain only the duty to promote their well-being by their advice and protection.
By Ingravescentem aetatem, issued motu proprio, Paul VI limits entry into the Conclave and voting to those cardinals who have not yet reached 80 years of age.
Paul VI limits the number of electors to 120.
Paul VI's Apostolic Constitution Romano Pontifici eligendo promulgates the general law for the death and election of the Pope and the governance of the Holy See during the vacancy. It consolidates the changes made during his pontificate and retains the 2/3rds + 1 norm instituted by Pope Pius XII.
Pope John Paul II promulgates the revised Code of Canon Law. As with the 1917 Code, the general law on the death and election of a Pope remains in particular law separately promulgated.
Pope John Paul II promulgates the apostolic constitution Universi Dominici Gregis. He re-establishes the 2/3rd majority rule, and allows a simple majority under some circumstances of a difficult election. He also establishes the new Domus Sanctae Martha residence in the Vatican for the exclusive use of the electors during the Conclave.
Pope Benedict XVI issues an Apostolic Letter De Aliquibus Mutationibus revoking the provision of Universi Dominici Gregis which allowed the Electors to break a deadlock by voting to elect by an absolute majority. The traditional procedure by which a 2/3rds majority is required to elect a Pope must be observed throughout the Conclave.