The Vatican said over the weekend that the conclave to choose the successor to Pope Benedict could start before 15 March if enough cardinals are in Rome to elect him.

Vatican spokesman Fr Federico Lombardi said church rules that regulate the timing of conclaves could be "interpreted" differently this time because of the unique circumstances of Benedict's historic resignation.

He had said earlier that the conclave would start between 15-20 March according to existing rules.

But Fr Lombardi said on Saturday that events could move more quickly since the church was dealing with an announced resignation and not a sudden papal death.

RTÉ Religious and Social Affairs Correspondent Joe Little looks at the process to choose Pope Benedict XVI’s successor.

Cardinals who are aged 80 or over when the Pope stands down on 28 February will not be eligible to vote.

But the 92 non-voting "Princes of the Church" are encouraged to attend important general meetings, called "congregations", of the 209-strong College of Cardinals in the weeks leading up to the election.

A full turnout of the privileged Cardinal-Electors would see 117 of them attend mass in St Peter's Basilica on the opening morning of the conclave.

The term comes from the Latin cum, "with", and clavis, "key", and describes the custom of the Cardinals locking themselves into a room while they are voting.

The practice was initiated in 1274 to prevent a repetition of events three years earlier when a mob locked up and eventually attacked the assembled College of Cardinals because they had failed to elect a Pope after a convention that lasted over two-and-a-half years!

Before today's Cardinals gather in the Vatican's Sistine Chapel, there is no formal declaration of candidates.

Each Cardinal-Elector will cast a maximum of four ballots per day.

The exception is the first day, following the Conclave Mass in St Peter's, when only one round of voting takes place.

The 2005 Conclave - at which Pope Benedict XVI was elected - saw the scrapping of the age-old practice of cramming each Cardinal-Elector into a cell in the part of the Apostolic Palace immediately adjoining the Sistine Chapel.

Groups of Electors are now ferried in hermetically-sealed buses between the Chapel and modern living quarters in Domus Santa Marthae (St Martha's House).

Because it is also located in the grounds of the Vatican City State, they are assured of maximum privacy in one of the world's most secretive electoral processes.

A winner emerges when two-thirds of the votes of those present are cast for him. After each vote, ballot papers are burned in the chapel stove.

Chemicals are mixed with them to produce black smoke when the vote is inconclusive and white smoke when a new Pope has been chosen.

Down the ages, waiting churchmen, pilgrims, onlookers and journalists have gathered in and around St Peter's Square to scrutinise the inelegant chimney for the ancient signals.

Shortly after white smoke emerges, the identity of the new Pope is announced to the world from the balcony above the main entrance to St Peter's Basilica.

The Latin words "Habemus Papam" (We have a Pope) are followed by the name the new Pontiff has taken and then his baptismal name.

And the Roman crowd explodes with relief, joy and some apprehension about what may lie ahead for him and the world's 1.2bn Catholics.