In the early 19th Century in rugby union football, the ball became dead in the field of play only by mutual consent of opponents. A player carrying the ball and held by opponents would say, "Held!", and his opponent would say, "Have it down." That is, the ballcarrier would declare himself fairly held, unable to advance, and an opponent would call on him to put the ball down, initiating the scrimmage. In modern rugby league, this is called a tackle and each team has six tackles to score; if they fail then possession changes over to the other team. The rule was established at four tackles in 1966 and was changed to six tackles in 1972. In American football, the concept of the act of having the ball down gave rise to "down" as the condition of the player so obligated, and the ball carrier could call for a "down" voluntarily. Although NCAA rules have effectively abolished this (as the ball carrier dropping to the ground immediately ends the play), other codes for North American football, such as the NFL, still allow (as one way for the ball to become dead) for the runner to cry "down". The rule is rarely used, despite having practical advantages over the preferred method of intentional downing, the kneel. Eventually the rules officially applied the word to include all of the action from the time the ball was put into play (whether by snap or free kick) until it became dead. The system of downs, in terms of a set number of plays to advance the ball a certain number of yards, were originally devised by Walter Camp and introduced to the game at the college football level in 1880, when Camp was still a player of the game. Camp's original system gave teams three downs to advance the ball five yards or else lose possession of the ball, a proposal meant to reduce sandbagging. Early in the 20th century, as the forward pass was added to the game and kicking rules became more restri ctive, a fourth down was added, and the requirement was doubled to 10 yards. The system of downs was introduced to Canadian football in 1903, where the Burnside rules imposed a ten yards in three downs requirement (like Walter Camp's early rules); those criteria remain in Canadian football to the present da