Before the closed-door conclave to elect Pope Francis' successor, Catholic cardinals have gathered in the Vatican for a final Mass and exhortation.

Catholic cardinals are gathering at the Vatican to elect the successor to Pope Francis, who died on April 21. Voting will take place in the Sistine Chapel, with ballots continuing in succession until any candidate secures two-thirds of the available votes.

Here's how it works, a list of contenders to become the next pope and how many conclaves there have been in the last 100 years.