A linebacker would best be defined as baking a cake using a defensive back and a defensive lineman as the two ingredients. A linebacker must possess the agility of a defensive back and the strength and toughness of a defensive lineman. A great linebacker must be able to turn and run with a receiver in coverage as well as shed blocks from offensive lineman that can out-weigh them by 60-70 pounds. A linebacker must also possess the most knowledge of any defender as they have to be able to recognize offensive formations and determine if a play is a run or a pass in only a split second.
In the 4-3 defense there are four down linemen and three linebackers. The middle linebacker is designated “Mike” and two outside linebackers are designated “Sam” and “Will” according to how they line up against the offensive formation. If there is a strong call, the linebacker on the strongside is called “Sam”, while the linebacker on the weakside is called “Will”. The outside linebacker’s job is to cover the end to make sure a run doesn’t escape, and to watch the pass and protect from it. The middle linebacker’s job is to stop runs between the tackles and watch the entire field to see the play develop. On pass plays, the linebackers’ responsibilities vary based upon whether a man or zone coverage is called. In a zone coverage, the linebackers will generally drop into hook zones across the middle of the field. However, some zones will send the outside linebackers into the flats (area directly to the left and right of the hash marks, extending 4-5 yards downfield). In a man-to-man call, the “Sam” will often cover the tight end with help from a safety over the top, while at other times, the “Sam” and “Will” will be responsible for the first man out of the backfield on their side of the center, with the “Mike” covering if a second man exits on that side of the field.
In the 3-4 Defense there are three lineman playing the line of scrimmage with four linebackers backing them up, typically two outside linebackers and two inside linebackers. The weak side inside linebacker is typically called the “Will,” while the strong side or middle inside linebacker is called the “Mike”. “Sam” is a common designation for strong outside linebacker, while the other position is usually called “Jack” and is often a hybrid DE/LB. Usually, teams that run a 3-4 defense look for college defensive ends that are too small to play the position in the pros and not quite fluid enough to play outside linebacker in a 4-3 defense as their “Jack” linebacker.
The idea behind the 3-4 defense is to disguise where the fourth rusher will come from. Instead of the standard four down-linemen in the 4-3, only 3 players are clearly attacking on nearly every play. A key for running this defense successfully is having a defensive front of three large defensive linemen who command constant double teams. In particular, the nose tackle, who plays over the offensive center, must be able to hold ground and to occupy several offensive blockers in order to allow the linebackers to make plays. The focus of the 3-4 defensive line is to occupy offensive linemen thus freeing the linebackers to tackle the running back or to rush the passer or otherwise drop into pass coverage.
Generally, both outside linebackers can rush the passer and play the run. Outside linebackers in the 3-4 defense tend to be larger in comparison to linebackers in a base 4-3 defense. They are also often players who would play DE in a 4-3 defense as situational pass rushing specialists but who otherwise may not fit the expected role of a DE being somewhat smaller in size. Outside linebackers should be able to drop into pass coverage, rush the passer or read and react. When it comes to the inside linebackers, one is generally a run stuffing player who is better able to handle offensive linemen and stop running backs when the offense features a running play, while the other is often a smaller, faster player who excels in pass coverage. However, the smaller or cover LB should also be able to scrape and plug running lanes decently.
The design concept of the 3-4 defense is to confuse the offensive line in their blocking assignments, particularly in pass blocking, and to create a more complex read for the quarterback. Many 3-4 defenses have the ability to quickly hybrid into a 4-3 on the field.