4-3 defense

In American football, a 4–3 defense is a defensive alignment consisting of four down linemen and three linebackers. It is probably the most commonly used defense in modern American football and especially in the National Football League. NFL teams that use the 4–3 defense as of 2011 include the Cleveland Browns, Philadelphia Eagles, New York Giants, Atlanta Falcons, Oakland Raiders, St. Louis Rams, Minnesota Vikings, Chicago Bears, Carolina Panthers, Cincinnati Bengals, Indianapolis Colts, Jacksonville Jaguars, Tennessee Titans, Detroit Lions, New Orleans Saints, Tampa Bay Buccaneers, New England Patriots, Seattle Seahawks, and Denver Broncos. The Broncos returned to the 4-3 with ith the hiring of John Fox as head coach.



The invention of the 4–3 is often attributed to legendary coach Tom Landry, in the 1950s, while serving as the Defensive Coordinator of the New York Giants, as a way to stop Hall of Fame running back Jim Brown. Others attribute the creation of the 4–3 to Chicago Bears Hall of Fame linebacker, Bill George. It has also been said that the 4–3 defense was a creation of Garrard "Buster" Ramsey, the Defensive Coach of the Detroit Lions teams in the 1950s.

In the original version of the 4–3, the tackles lined up over the offensive guards and the ends lined up over the offensive tackles, with the middle linebacker over the center and the other linebackers outside the ends. In the mid-1960s Hank Stram developed a popular variation, the "Kansas City Stack", which shifted the strong side defensive end over the tight end, stacked the strongside linebacker over the tackle, and shifted the weakside tackle over center. At about the same time the Cleveland Browns frequently used a weakside shift. The Dallas Cowboys coach Tom Landry developed a "flex" variation, that moved standout lineman Randy White all over and set two of the linemen a half-step farther back from the offensive linemen. Now every team has its own variations.

Defensive Line
There are two defensive tackles in the 4–3 scheme. Some teams especially in the NFL do have a nose tackle in this scheme, but most of them do not. In a traditional 4–3 defensive set, there is no nose tackle. Instead there is a left and right defensive tackle. When teams don't have a nose tackle, the tackles lined up over the offensive guards. The nose tackle is generally slightly larger and stronger and plays a shade or head-up technique which means he lines up on either outside shoulder of the center or in the middle of his body depending on which way the strength of the play is going. The nose tackle's primary job is to stop the run and take on the double team (which is getting blocked by both the center and the weak-side or pulling guard) thus freeing up the linebackers to make a play. The second defensive tackle (simply referred to as the defensive tackle, under tackle or three tech) is generally a bit quicker and faster than the nose tackle, ideally weighing close to 300 pounds but quick-footed enough to shoot through a gap at the snap. He plays a three technique meaning he lines up on the outside shoulder of the strong side offensive guard. The job of a three tech is to: prevent the run, keep the guard off linebackers, and rush the quarterback on pass plays.
 * Defensive Tackles

Teams that want to use a standard 4–3 scheme often face a dilemma: there aren't enough great defensive ends to go around. Players like Julius Peppers or Jevon Kearse come along about once per year in the draft.
 * Defensive Ends

The defensive end's primary role in the 4–3 defense is to get to the quarterback and create pressure. The 4–3 DE's are the smallest of all of the defensive lineman due to their emphasis of speed over strength. They still need to be strong enough to fight their way past offensive tackles, yet quick enough to pursue the running backs on runs to the outside. Ideal 4–3 defensive ends are athletic and agile and their strength is getting up the field quickly and they usually weigh between 260 and 275 pounds. Right ends, who line up against the offensive left tackle and attack the quarterback from the blind side, are usually the best athletes on the line, combining a 275-pound body with incredible quickness and agility to outflank blockers who are bigger and heavier. Defensive ends generally play the 1 gap technique, though will occasionally be forced to play a 2 gap in the event of a TE pinching in to block on run plays. In most schemes, they are also responsible for keeping the quarterback from rolling out of the pocket to make big running gains.

Linebackers
There is only one inside linebacker in the 4–3 scheme, so he is called the Middle linebacker, sometimes known as the “Mike” linebacker. He must be as smart as he is athletic, acts as the “quarterback of the defense” and is often the defensive leader. The primary responsibility of the “Mike” is to stop the run, though he will often be asked to fall back in zone coverage in pass protection; man to man pass coverage has him assigned to the fullback typically. The MLB is often the largest and strongest of all of the linebackers.
 * Middle Linebacker

The 4–3 defense relies on having a sure tackler at the middle linebacker spot. Most notably, Tony Dungy's “Tampa Cover 2” scheme makes high demands on the MLB, requiring him to have above-average speed, and additional skills to be able to read the play and either maintain his central position to help the outside linebackers cover short passes, drop behind the linebackers in coverage and protect the zone of the field behind the outside linebackers from 11–20 yards out, or run up to the line of scrimmage to help assist in stopping the runs.

As in the 3–4 defense there are two outside linebackers in the 4–3. These outside backers are known as the Strong-Side and Weak-Side Linebackers. The Strong-Side, or “Sam” linebacker, is so named because he typically sticks to the strong side of the defense, across from the TE. The “Sam” does his fair share of blitzing, however he also needs to play the run and will usually be relied upon to cover the tight end or potentially a back out of the backfield. The Weak-Side, or “Will” linebacker, will generally play on the weak side and has more freedom than the other LBs, often blitzing the QB or guarding against the screen.
 * Outside Linebackers

Secondary
The 4-3 defense generally uses four defensive backs. Two of these are safeties, and two of them are cornerbacks. A cornerback's responsibilities vary depending on the type of coverage called.

Coverage is simply how the defense will be protecting against the pass. The corners will generally line up 3 to 5 yards off the line of scrimmage, generally trying to "Jam" or interrupt the receivers route within the first 5 yards. A corner will be given one of two ways to defend the pass (with variations that result in more or less the same responsibilities): zone and man-to-man. In zone coverage, the cornerback is responsible for an area on the field. In this case, the corner must always stay downfield of whomever it is covering while still remaining in its zone. Zone is a more relaxed defensive scheme meant to provide more awareness across the defensive secondary while sacrificing tight coverage. As such, the corner in this case would be responsible for making sure nobody gets outside of him, always, or downfield of him, in cases where there is no deep safety help. In man coverage, however, the cornerback is solely responsible for the man across from him, usually the offensive player split farthest out.

The free safety is responsible for reading the offensive plays and covering deep passes. Depending on the defensive call, he may also provide run support. He is positioned 10 to 15 yards behind the line of scrimmage, toward the center of the field. He provides the last line of defense against running backs and receivers who get past the linebackers and cornerbacks. He must be a quick and smart player, capable of making tackles efficiently as well as reading the play and alerting his team of game situations. The strong safety is usually larger than the free safety and is positioned relatively close to the line of scrimmage. He is often an integral part of the run defense, but is also responsible for defending against a pass; especially against passes to the tight-ends.