Tackle is a playing position in American and Canadian football. Historically, in the one-platoon system a tackle played on both offense and defense. In the modern system of specialized units, offensive tackle and defensive tackle are separate positions, and mere "tackle" refers to an offensive tackle. The offensive tackle (OT, T) is a position on the offensive line, left and right. Like other offensive linemen, their job is to block: to physically keep defenders away from the offensive player who has the football and enable him to advance the football and eventually score a touchdown. The term "tackle" is a vestige of an earlier era of football in which the same players played both offense and defense.