2 - Secret conclave
The election of a pope is conducted in conditions of secrecy unique in the modern world.
The cardinals are shut away in the Vatican until they reach agreement - the meaning of the word conclave indicating that they are literally locked up "with a key".
The election process can take days. In previous centuries it has gone on for weeks or months and some cardinals have even died during conclaves. In one case, it took nearly three years.
The process is designed to prevent any of the details of the voting emerging, either during or after the conclave. The threat of excommunication hangs over anyone tempted to break this silence.
Before the voting begins in the Sistine Chapel, the entire area is checked by security experts to ensure there are no hidden microphones or cameras.
Once the conclave has begun, the cardinals eat, vote and sleep within closed-off areas until a new pope has been chosen.
They are allowed no contact with the outside world - barring a medical emergency. All radios and television sets are removed, no newspapers or magazines are allowed in, and mobile phones are banned.
Two doctors are allowed into the conclave, as well as priests who are able to hear confessions in various languages and housekeeping staff.
All these staff have to swear an oath promising to observe perpetual secrecy, and undertake not to use sound or video recording equipment.
The cardinals are on their own when the votes are cast in the Sistine Chapel.
The practice of holding secret conclaves grew up in an attempt to force the cardinals to reach agreement quickly. If they took too long, those waiting outside started to reduce their rations.
But it was soon seen as a way of allowing the cardinals to concentrate on the task without distractions and of stopping governments from trying to influence the election.
At earlier conclaves the cardinals had to endure Spartan conditions in makeshift "cells" close to the Sistine Chapel. They slept on hard beds and were issued with chamber pots. Now they will be lodged in a modern, hotel-style building within the Vatican complex.
Although not allowed to vote, the over-80s can exert an influence on the election in the preparatory meetings before the conclave.
The rules of secrecy have been extended to the over-80s since 1978, when detailed accounts of voting were assumed to have emerged because this precaution was not in place.
The requirement for secrecy continues after the election, unless the new pope rules otherwise.
The precise number eligible to vote changes as cardinals die or reach the age of 80, and are gradually replaced by younger men.
Therefore the make-up of the College of Cardinals is constantly changing, and the balance of power between different factions shifts over time.