During the second half of the 20th century, the rules changed further. In 1966, rugby league officials borrowed the American football concept of downs: a team was allowed to retain possession of the ball for four tackles (rugby union retains the original rule that a player who is tackled and brought to the ground must release the ball immediately). The maximum number of tackles was later increased to six (in 1971), and in rugby league this became known as the six tackle rule.