Pope Pius X had issued two apostolic constitutions on the subject of papal conclaves. The first, Commissum Nobis of 20 January 1904, eliminated any secular monarch's claim to a veto over a candidate for election. It established that anyone who attempted to introduce a veto in the conclave would incur automatic excommunication. For the first time in centuries the cardinals alone would make the choice.
Balloting
The conclave itself assembled in the Sistine Chapel on 31 August. From the beginning of the conclave, it was clear that there were only three possible winners. Domenico Serafini, a Benedictine and assessor at the Holy Office, won the support of the Curia to continue Pius X's anti-modernist campaign as his chief priority. However, many other cardinals, such as Carlo Ferrari and Desiré Merciér, believed that a Pope with a different focus was needed and supported the Archbishop of PisaPietro Maffi, considered very liberal but tainted by being close to the House of Savoy. Giacomo della Chiesa, Archbishop of Bologna, stood intermediate between Maffi and Serafini, but in the early ballots he was equal with Maffi and seemed to be winning some support from conservative factions. Della Chiesa drew ahead by five votes after the fourth ballot, and once it became clear Maffi had no hope whatsoever of gaining two-thirds of the votes, Serafini became Della Chiesa's opponent. By 3 September 1914, on the tenth ballot, all of Maffi's supporters had switched to Della Chiesa, who was elected pope. He took the name Benedict XV. Reportedly Della Chiesa had been elected by one vote. According to the rules in force at the time, the ballot papers had a numbering on the reverse side, so that, if the election was decided by only one vote, it could be checked whether or not the elected person had voted for himself, in which case the election would be void. According to that account, Cardinal Rafael Merry del Val, who had been Pius X's Secretary of State, insisted that the ballots be checked to ensure that Della Chiesa had not voted for himself – he had not. When the cardinals offered their homage to the new pope, Benedict allegedly said to Merry del Val, "The stone that the builders rejected has become the cornerstone." To which the unabashed Merry del Val replied with the next verse of Psalm 118: "This is the Lord’s doing; it is marvelous in our eyes." Cardinal Merry del Val wasn't reappointed as Secretary of State by the new Pope, but was named Secretary of the Supreme Sacred Congregation of the Holy Office.