Over time, the typically fast back who played fullback, on offense and defense, was replaced by a heavier one who presented a greater threat to run with the ball more or less straight ahead, and to tackle his opponent trying to do the same. For this purpose, the player tended to be placed closer to the line than previously—often as far forward as the halfbacks (the offensive backs then forming a letter T, sometimes called a "straight T", later to distinguish it from slight variants wherein other backs did not form a line perpendicular to that from the quarterback) or even farther forward. But the position kept being called "fullback". On defense, this necessitated another position name for the fast back who played farthest back. The phrase "goal-tend" was used for a time near the start of the 20th century before settling upon "safety man" or simply "safety" (S), representing the last defense against a breakaway play, and the position from which to field opposing kicks. Approximately, on defense, the quarterback and fullback exchanged positions from what they'd been playing on offense, and the quarterback became the safety.