Underboss

From The Godfather Wiki

The Underboss (also known as sotto capo) is the second most important position in a crime family, directly beneath the Don himself. The Underboss is usually the son of the current Don. The Underboss, usually appointed by the Boss, is the second in command of the family. The Underboss is in charge of all of the Capos, who are controlled by the Boss. The Underboss is usually first in line to become Acting Boss if the Boss is imprisoned or dies. There are normally no more than two or three Underbosses in a family, if there's more than one at all.