Accessus is a term applied to the voting in conclave
Papal conclave
A papal conclave is a meeting of the College of Cardinals convened to elect a Bishop of Rome, who then becomes the Pope during a period of vacancy in the papal office. The Pope is considered by Roman Catholics to be the apostolic successor of Saint Peter and earthly head of the Roman Catholic Church...

 for the election of a pope
The Pope is the Bishop of Rome, a position that makes him the leader of the worldwide Catholic Church . In the Catholic Church, the Pope is regarded as the successor of Saint Peter, the Apostle...

, by which a cardinal
Cardinal (Catholicism)
A cardinal is a senior ecclesiastical official, usually an ordained bishop, and ecclesiastical prince of the Catholic Church. They are collectively known as the College of Cardinals, which as a body elects a new pope. The duties of the cardinals include attending the meetings of the College and...

 changes his vote and accedes to some other candidate. Accessus voting was first used in the papal conclave, 1455
Papal conclave, 1455
The papal conclave from April 4-8, 1455 elected Alfons Borja Pope Callixtus III following the death of Pope Nicholas V. The conclave was the first in the Apostolic Palace, the site of all but five papal conclave thereafter...

. The procedure was likely adopted from the Roman Senate
Roman Senate
The Senate of the Roman Republic was a political institution in the ancient Roman Republic, however, it was not an elected body, but one whose members were appointed by the consuls, and later by the censors. After a magistrate served his term in office, it usually was followed with automatic...

where an acceding Senator would physically move to join the proponents of a proposal.

When the votes of the cardinals have been counted after the first balloting and the two-thirds majority has fallen to none of those voted for, at the following vote opportunity is granted for a cardinal to change his vote, by writing "Accedo domino Cardinali", mentioning one of those who have been voted for, but not the cardinal for whom he has already voted. If he should not wish to change his vote, the cardinal can vote "Nemini" ( "for no one" ). If these supplementary votes of accession, added to those a candidate has received, equal two-thirds of the total vote, then there is an election. If not, the ballots are burned, and the usual ballot takes place the next day.

Election by accessus was only possible because, until the mid-20th century, the ballots used by each Cardinal were identified by a text of scripture in the back side. When a Cardinal decided to use the right of accession, his two ballots had to "be compared and identified by the text on the reverse face of the ballot, so as to prevent a double vote for the same candidate by any elector" .

Thus, voting by accession eliminated the secrecy of the first ballot.

Voting by accessus was prohibited by the Cardinal Dean in the 1903 Conclave and later this form of election was suppressed from the Church's legislation.
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