Defensive Formation – 4-3 Defense Diagram

"This base defense consists of four defensive linemen, three linebackers, and four defensive backs (two safeties, two corners). Against two-receiver offensive sets, this formation is effective against the run and the pass. In the original 4–3, defensive tackles would line up opposite the offensive guards, and defensive ends on the outside shoulders of the offensive tackles. On passing downs, the Mike (middle linebacker) is often responsible to cover any running backs, the Sam (strong-side linebacker) covers the Tight End, and the Will (weak-side linebacker) either covers a back or blitzes in an attempt to sack the quarterback. Though first used as a base defense by the New York Giants in 1956, plenty of teams experimented with it during the 1950s, and thus there are multiple claimed inventors of this defense. There are several different variations of the 4–3 defense such as the 4-3 under defense, 4-3 over defense, 4-3 umbrella defense, 4-3 swim defense, and 4-3 slide defense." [List of formations in American football. Wikipedia]
This example diagram was redesigned from the Wikimedia Commons file: 4-3 green.svg.
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The American football positions diagram example "Defensive Formation - 4-3 defense" was created using the ConceptDraw PRO diagramming and vector drawing software extended with the Football solution from the Sport area of ConceptDraw Solution Park.

"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. ...
In the original version of the 4–3, the tackles lined up over the offensive guards and the ends lined up on the outside shoulder of 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, in order to take advantage of the quickness of his Hall of Fame tackle, Bob Lilly. In Tom Landry's original 4–3 defenses (4-3 Inside and 4-3 Outside), both defensive tackle were flexed. In the "flex", on a pro set right, with defensive keys showing a run to the right, the right defensive tackle would be flush on the line and was supposed to penetrate. The right defensive end and left defensive tackle were flexed two feet off the line of scrimmage, the right defensive end now head on with the left offensive tackle (i.e. a 4-2-2-5 front instead of the more common 5-2-2-5 front). This gave the defense a "zig zag" look unlike any other of its day. More modern versions of the 4-3 include the Tampa 2 scheme and the 4-3 slide." [4–3 defense. Wikipedia]
The American football positions diagram example "Defensive play - Under front" was created using the ConceptDraw PRO diagramming and vector drawing software extended with the Football solution from the Sport area of ConceptDraw Solution Park.

"In American football, each team has eleven players on the field at one time. The specific role that a player takes on the field is called their position. Under the modern rules of American football, teams are allowed unlimited substitutions, that is teams may change any number of players after any play. This has resulted in the development of three "platoons" of players, the offense (the team with the ball, who is trying to score), the defense (the team trying to prevent the other team from scoring, and to take the ball from them), and the special teams (who play in kicking situations). Within those platoons, various specific positions exist depending on what the player's main job is." [American football positions. Wikipedia]
The American football positions diagram example "3-4 Defense" was created using the ConceptDraw PRO diagramming and vector drawing software extended with the Football solution from the Sport area of ConceptDraw Solution Park.