When Roman Catholic cardinals enter the secret conclave to choose a new pope on Tuesday they will be following a ritual dating to the 13th century, when papal elections could last for years and some cardinals died during the gruelling process.

The term conclave comes from the Latin for "with a key" and refers to the practice of locking cardinals away from the world's prying eyes to allow them to choose a new pope without outside interference.

The 115 cardinal-electors will disappear from public view on Tuesday to vote in the Sistine Chapel for a successor to Pope Benedict as leader of the world's 1.2 billion Roman Catholics. 

A puff of white smoke from a Vatican chimney, signifying a decision has been reached, is expected within a few days if the conclaves of the past 100 years are any guide. 

It was not always that straightforward.