A halfback is the holder of an offensive position in American football whose duties involve lining up in the backfield and carrying the ball on most running plays in formations such as the veer variant of the wishbone. When the principal ball carrier lines up deep in the backfield, and especially when that player is placed behind another player (usually a blocking back), as in the I formation, that player is instead referred to as a tailback (see History below).^, Historically, from the 1870s through the 1950s, the halfback position was both an offensive and defensive position. In the related sport of Canadian football, halfback is usually a defensive rather than offensive position since the 1980s, though used to also refer to an offensive position similar to a slotback that could line up off the tight end or behind the quarterback. Some systems require the halfback be proficient at throwing the ball downfield as well. A halfback famous for his passing ability was Marcus Allen, who threw 6 touchdowns in the NFL.