Pope Nicholas II in the context of Duchy of Apulia

Robert Guiscard (/ɡiːˈskɑːr/ ghee-SKAR, Modern French: [ʁɔbɛʁ ɡiskaʁ]; c. 1015 – 17 July 1085), also referred to as Robert de Hauteville, was a Norman adventurer remembered for his conquest of southern Italy and Sicily in the 11th century.

Robert was born into the Hauteville family in Normandy, the sixth son of Tancred de Hauteville and his wife Fressenda. He inherited the County of Apulia and Calabria from his brother in 1057, and in 1059 he was made Duke of Apulia and Calabria and Lord of Sicily by Pope Nicholas II. He was also briefly Prince of Benevento (1078–1081), before returning the title to the papacy.

The County of Apulia and Calabria (Latin: Comitatus Apuliae et Calabriae), later the Duchy of Apulia and Calabria (Latin: Ducatus Apuliae et Calabriae), was a Norman state founded by William of Hauteville in 1043, composed of the territories of Gargano, Capitanata, Apulia, Vulture, and most of Campania. It became a duchy when Robert Guiscard was raised to the rank of duke by Pope Nicholas II in 1059.

The duchy was disestablished in 1130, when the last duke of Apulia and Calabria, Roger II, became King of Sicily. The title of duke was thereafter used intermittently as a title for the heir apparent to the Kingdom of Sicily.

The selection of the pope, the bishop of Rome and supreme pontiff of the Catholic Church, prior to the promulgation of In nomine Domini in 1059 varied throughout history. Popes were often putatively appointed by their predecessors or by political rulers. While some kind of election often characterized the procedure, an election that included meaningful participation of the laity was rare, especially as the popes' claims to temporal power solidified into the Papal States. The practice of papal appointment during this period would later result in the putative jus exclusivae, i.e., the claimed but invalid right to veto the selection that Catholic monarchs exercised into the twentieth century.

The absence of an institutionalized procedure of papal succession facilitated religious schism, and the Catholic Church currently regards several papal claimants before 1059 as antipopes. Further, the frequent de facto requirement of political approval of elected popes significantly lengthened periods of sede vacante, i.e., transitional vacancy of the papacy, and weakened it. In 1059, Pope Nicholas II succeeded in limiting future papal electors to the College of Cardinals in In nomine Domini, instituting standardized papal elections that eventually developed into the procedure of the papal conclave.

In nomine Domini (Latin for In the name of the Lord) is a papal bull written by Pope Nicholas II. The bull was issued on 13 April 1059 and caused major reforms in the system of papal election, most notably establishing the cardinal-bishops as the sole electors of the pope, with the consent of minor clergy.

The 1061 papal election was held on 30 September 1061 in San Pietro in Vincoli ("Saint Peter in Chains") in Rome, following the death of Pope Nicholas II. In accordance with Nicholas II's bull, In Nomine Domini, the cardinal bishops were the sole electors of the pope for the first time in the history of the Roman Catholic Church. Bishop Anselmo de Baggio of Lucca, a non-cardinal and one of the founders of the Pataria, was elected Pope Alexander II and crowned at nightfall on 1 October 1061 in San Pietro in Vincoli Basilica because opposition to the election made a coronation in St. Peter's Basilica impossible.