The libero player was introduced internationally in 1998, and made its debut for NCAA competition in 2002. The libero is a player specialized in defensive skills: the libero must wear a contrasting jersey color from their teammates and cannot block or attack the ball when it is entirely above net height. When the ball is not in play, the libero can replace any back-row player, without prior notice to the officials. This replacement does not count against the substitution limit each team is allowed per set, although the libero may be replaced only by the player whom he or she replaced. Most U.S. high schools added the libero position from 2003 to 2005.