The development of the forward pass in American football shows how the game has evolved from its rugby roots into the distinctive game it is today. Illegal and experimental forward passes had been attempted as early as 1876, but the first legal forward pass in American football took place in 1906, after a change in rules. Another change in rules occurred on January 18, 1951, which established that no center, tackle, or guard could receive a forward pass (unless such a player announces his intent to the referee beforehand that he will be an eligible receiver, called a tackle-eligible play). Today, the only linemen who can receive a forward pass are the ends (tight ends and wide receivers). Current rules regulate who may throw and who may receive a forward pass, and under what circumstances, as well as how the defensive team may try to prevent a pass from being completed. The primary pass thrower is the quarterback, and statistical analysis is used to determine a quarterback's success rate at passing in various situations, as well as a team's overall success at the "passing game."