White smoke: pope. Black smoke: nope. Live video of chimney

Grant Burningham

The Sistine Chapel may be best known for its elaborate ceiling painted by Michelangelo, but for much of this week the world's attention will be focused on an ungainly addition to its roof: a copper spout from which a telltale wisp of smoke will alert observers that the Roman Catholic Church has chosen a new leader.

The College of Cardinals, 115 men gathered to pick a new pope, use the chimney as a form of communication known to humans long before Christ's birth. Papal voting often goes several rounds at the conclave, a Latin word meaning "without keys," dating to a time when the vote was so contentious, it required locking cardinals in a room until they made a decision.

At the end of each voting round, the ballots are burned. The color of smoke coming from the chimney tells the results: black if no candidate reaches a two-thirds majority, white if there's a winning candidate.

The smoke signal produces elation and sometimes confusion. When Cardinal Ratzinger emerged as the pope in 2005, a dark smudge appeared from the chimney, and the gathered masses waited as it seemed to get more pale. Finally, the bells of St. Peter brought confirmation.

The smoke can also be obscured by overcast skies and rain, and can be hard to read against the night sky, as it was for Pope John Paul II in 1978. The Vatican says it has a plan this time around to avoid any doubt: spotlights.

Traditionally, black smoke was created by adding wet straw to burning ballots. But in 2005 science stepped in to add some, well, clarity. A chemical is now added to make black smoke.

Pope Benedict XVI was elected after just four rounds, but this time a lot of black smoke is expected before the white, even if Cardinal Dolan of New York, who took to a more modern communication medium, Twitter, to suggest there may be a new pontiff by Thursday, is correct.