Linebackers are primarily run stoppers and after that have pass coverage. The middle linebacker lines up in between the offensive guards. This player's main job is to stop the run; to be most effective, the middle linebacker should be the team's best tackler and be able to play from sideline to sideline. Pass coverage is a short zone on any backs coming through the middle of the line. When blitzing, the middle linebacker takes the "A gap" opposite of the nose tackle, becoming a "de facto" fourth lineman. The outside linebackers line up behind the defensive ends. They have pass coverage on running backs coming out of the backfield to the flats, can cover slotbacks but may switch with defensive backs. Defensive backs look for the pass first and then the run. They have coverage responsibilities for tight and split ends, but may cover slotbacks and leave tight ends for the outside linebackers.