If no clear choice has emerged after three days, balloting is suspended for 24 hours to allow cardinal electors time to reflect. Since 1831, no conclave has lasted more than four days.

The end of each round of balloting will be signaled by smoke emanating from a stovepipe chimney atop the Sistine Chapel. Black smoke -- fumata nera in Italian -- indicates an inconclusive vote, while white smoke -- fumata bianca -- will signify that a new pope has been elected, as will the ringing of the bells of St. Peter's Basilica.

As Vatican City begins to fill with the faithful on Wednesday morning, the weather forecast calls for temperatures in the mid-60 degrees Fahrenheit, with midday showers expected, according to the Italian Meteorological Service.

More than 25,000 people are expected to gather into St. Peter's Square on Wednesday, along with crowds of people spilling over into the nearby roads.

About 4,000 security personnel are on hand, along with about 1,000 volunteers. And about 5,300 journalists from around the world will be present, waiting for news from the conclave.