Historically, from the 1870s through the 1950s, the halfback position was both an offensive and defensive position. Now that most offensive formations have only one or two running backs, the original designations do not mean as much, as the fullback is now usually a lead blocker (technically a halfback), while the halfback or tailback (called such because he stands at the "tail" of the I) lines up behind the fullback. There has also been a shift in most offense's dependence on halfbacks, as the quarterback is now generally considered the most essential part of a team. However, the average output of the halfback has not changed.