When 115 cardinals file into the Sistine Chapel on Tuesday, their scarlet vestments will be an outward symbol of their rank in the Roman Catholic Church as electors of the pope.
The "Princes of the Church" normally wear a black cassock with red buttons, but don the scarlet cassocks, birretas (skullcaps) and mozzettas (short capes) for the duration of the conclave.
The colour symbolises blood and the churchmen's willingness to die for their faith, metaphorically as the "body" of the Church until the new pope is elected.
The cardinals also wear a large ring, usually of sapphire.
Like the senators of the Roman Empire, who were considered "part of the emperor's body", cardinals are the "senate" of the Church and since the 12th century have been considered "part of the pope's body".
In Roman times, the title "cardinalis" was given to officers of the crown, army generals and prefects in Asia and Africa.
Within the Church, cardinals were originally members of the Rome clergy, below the bishop of Rome whom they elected.
Under Gregorian reforms in the 11th century, Pope Nicolas II clarified their status and gave them a ranking above other bishops of the Church.
In 1181, Rome's cardinals became the sole electors of the pope.
The Vatican newspaper on Sunday published a dress code for the cardinals covering the pre-conclave mass and the conclave itself.
The "note" in Sunday's Osservatore Romano stresses that vestments are outward signs of the Church's power denoting each clergyman's rank in the hierarchy from pope down to simple parish priest.
Until a new "instruction" in 1969, cardinals wore the galero, a big flat hat with 10 tassels on each side, to the ceremony that creates cardinals, known as a consistory.