White headwear is worn by the Pope and what other Roman Catholic clergy?

Question

What is the name of the long white garment worn underneath other vestments by clerics at religious ceremonies in many branches of Christianity?

Answer 1

The Canons Regular of the Order of Prémontré
The Pope wears a white camauro. The Canons wear a white biretta. The 3 cornered biretta, a hat worn by many other Roman Catholic clergy, is red for Cardinals, purple for Bishops and black for priests, deacons and seminarians. The retired Pope Benedict XVI, as at 1 March 2013, also wears white on his head.