Tomorrow, Catholic Cardinals from around the world will gather for the conclave to elect the successor to Pope Francis.

John Yang takes a look now at the high stakes undertaking.

White smoke from the Sistine Chapel's chimney, the signal that there's a new head of the world's largest Christian church.

It's the culmination of a centuries-old process that's shrouded in secrecy and traditions.

We are a church of 1.3 billion people around the world, and this is a fundamental question of

importance, both for all of the Catholics around the world, but also as a source of a moral voice on the world stage.

The procedure for electing a pope has been called a conclave since the Middle Ages.

The word conclave comes from two Latin terms, Qum meaning with Gavis meaning key.

And it refers to the fact that Cardinals are physically locked into a room.

It's now the Sistine Chapel and not allowed to come out, really, until they have picked a Pope.

And this is because in centuries past, they could become horribly deadlocked.

The famous instance was the conclave of the Terbo in the 13th century, where it

literally took them almost two and a half years to move from the death of one Pope to the

election of another. And that situation was simply found to be intolerable.

And so, Pope Gregory the 10th, who emerged from that prolonged conclave, established strict rules in hopes of avoiding a repeat.

The process continued to evolve.

The modern framework is spelled out in the apostolic constitution, Pope John Paul II, issued in 1996.

Conclave's must start within 15 to 20 days after a Pope's death or rarely resignation.

They're usually overseen by the Dean of the College of Cardinals, a senior cardinal elected by

Cardinal Bishops, the highest order of Cardinals, and confirmed by the Pope. In this conclave, only 135 of the

252 cardinals are permitted to participate, what are called cardinal electors. That's because only

those younger than 80 can vote, a limit set in 1970 by Pope Paul the 6th. The desire was to be sure

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