The Vatican’s cardinals gather in conclave today to begin electing the successor to the late Pope Francis—which means the world’s attention is on a chimney attached to the Sistine Chapel. Here puffs of black smoke mark voting rounds that fail to converge on a winner until the cardinals announce that they have come to an agreement by releasing a plume of white smoke,

Here's how the conclave creates black and white smoke and why the Catholic Church began using them to signal whether a new pope has been elected.

A total of 133 cardinal electors, all under the age of 80, are eligible to vote in this conclave — the highest number in history.