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In gridiron football, a lineman is a player who specializes in play at the line of scrimmage. The linemen of the team currently in possession of the ball are the offensive line, while linemen on the opposing team are the defensive line. A number of NFL rules specifically address restrictions and requirements for the offensive line, whose job is to help protect the quarterback from getting sacked for a loss, or worse, fumbling.  The defensive line is covered by the same rules that apply to all defensive players. Linemen are usually the largest players on the field in both height and weight, since their positions usually require less running and more strength than skill positions.
The offensive line consists of the center, who is responsible for snapping the ball into play, two guards who flank the center, and offensive tackle duo flanking these guards. In addition, a full offensive line may also include a tight end outside one or both of the tackles.
An offensive lineman's motion during a play is often limited to just a few quick steps (typically from a three-point stance) to establish position, followed by a wrestling match similar to sumo.  Offensive linemen thus tend to be the largest and strongest players on the field after nose tackle, with excellent agility and balance but limited straight-line running speed and stamina.
On some running plays, an offensive lineman will pull by backing out of his initial position and running behind the other offensive linemen to engage a defensive player beyond the initial width of the offensive line; in modern games this duty usually falls to guards.
When an offensive lineman knocks a player down on a block, leaving the defensive player lying flat on his back, it is known as a pancake block. 
When an offensive line has an equal number of men on either side of the center, it is known as a balanced line.  The interior offensive line consists of the center and guards. 
Offensive linemen are not eligible to catch forward passes, and are not allowed to advance more than two yards past the line of scrimmage at the time a pass is thrown, whether they are engaged with a defensive player or not. Ends (tight ends and wide receivers) are eligible to catch passes. One exception to this rule is whenever a tackle-eligible play is executed, but such a play must be announced by the referee beforehand.
On running plays, the primary job of the offensive line is to create space for the ball carrier to run, either by pushing all defensive players backwards past the line of scrimmage, or by pushing defensive players to the side to allow the ball carrier to run past them.
On passing plays, the offensive line is responsible for stopping defensive players from tackling the quarterback before he has thrown the ball. Stopping these players indefinitely is usually not possible, so the main objective of the offensive line is to slow them down, providing the quarterback with several seconds to identify an open receiver and throw the ball.
The defensive line consists of one or two defensive tackles and two defensive ends who play outside the defensive tackles. The defensive line works with the linebackers to try to control the line of scrimmage. The 4-3 defense, most commonly used in the NFL, employs two defensive tackles (and a defensive line of four men, with three linebackers behind them), while the 3-4 defense uses just a single defensive tackle, called the nose tackle (and a defensive line of three men, with four linebackers behind them). Defensive ends in a typical 3-4 have responsibilities more similar to a 4-3 defensive tackle than 4-3 defensive ends.
On running plays, the goal is to tackle the ball carrier. The defensive line attempts to maintain their original formation (even spacing without holes), but also to prevent any members of the opposing offensive line from successfully engaging the linebackers, who chase down the ball carrier. The defensive tackles are usually the most skilled run defenders on the team.
On passing plays, the defensive line tries to reach the quarterback. Ideally, the defensive players are able to tackle the quarterback for a loss (a sack), but in practice the quarterback will usually manage to throw the ball before an actual tackle is made; the goal is thus to put pressure on the quarterback as quickly as possible to force him to throw the ball before he can find an open receiver. Defensive ends are usually the most skilled pass rushers on the team. In order to increase the pressure on the quarterback, teams will often have players other than the defensive line attempt to tackle the quarterback; this is called a blitz.
Because the defense does not know whether the offense is attempting to run a passing play or a running play (or whether a quarterback will give up on an attempt to pass and instead run with the ball), they must balance passing and running strategies: running around offensive linemen and avoiding contact may allow faster pressure on a quarterback, but it also leaves a hole in the defensive line and frees an offensive lineman to engage a linebacker, enabling a big running play.
Defensive linemen, particularly defensive ends are typically lighter and faster than defensive tackles and offensive linemen. Defensive tackles weight averages between 260-310 lbs. 
A running back (RB) is a member of the offensive backfield in gridiron football. The primary roles of a running back are to receive handoffs from the quarterback to rush the ball, to line up as a receiver to catch the ball, and block. There are usually one or two running backs on the field for a given play, depending on the offensive formation. A running back may be a halfback, a wingback or a fullback. A running back will sometimes be called a "feature back" if he is the team's starting running back.
Linebacker (LB) is a playing position in gridiron football. Linebackers are members of the defensive team, and line up three to five yards behind the line of scrimmage and the defensive linemen. They are the "middle ground" of defenders, playing closer to the line of scrimmage than the defensive backs (secondary), but farther back than the defensive linemen.
The tight end (TE) is a position in American football, arena football, and Canadian football, on the offense. The tight end is often a hybrid position with the characteristics and roles of both an offensive lineman and a wide receiver. Like offensive linemen, they are usually lined up on the offensive line and are large enough to be effective blockers. On the other hand, unlike offensive linemen, they are eligible receivers adept enough to warrant a defense's attention when running pass patterns.
In gridiron football, blitzing is a tactic used by the defense to disrupt pass attempts by the offense. During a blitz, a higher than usual number of defensive players will rush the opposing quarterback, in an attempt either to tackle him or force him to hurry his pass attempt.
This is a glossary of terms used in Canadian football. The Glossary of American football article also covers many terms that are also used in the Canadian version of the game.
Strategy forms a major part of American football. Both teams plan many aspects of their plays (offense) and response to plays (defense), such as what formations they take, who they put on the field, and the roles and instructions each player are given. Throughout a game, each team adapts to the other's apparent strengths and weaknesses, trying various approaches to outmaneuver or overpower their opponent in order to win the game.
The 46 defense is an American football defensive formation, an eight men in the box defense, with six players along the line of scrimmage. There are two players at linebacker depth playing linebacker technique, and then three defensive backs. The 46 defense was originally developed and popularized with the Chicago Bears by their defensive coordinator Buddy Ryan, who later became head coach of the Philadelphia Eagles and Arizona Cardinals.
A formation in football refers to the position players line up in before the start of a down. There are both offensive and defensive formations and there are many formations in both categories. Sometimes, formations are referred to as packages.
Center or Centre (C) is a position in gridiron football. The center is the innermost lineman of the offensive line on a football team's offense. The center is also the player who passes the ball between his legs to the quarterback at the start of each play.
In American football, a zone blitz is a defensive tactic that sends additional players to rush the opposing team's quarterback, whilst also unexpectedly redirecting a supposed pass rushing player into pass coverage instead. This tactic also likely includes zone coverage.
Single set back is an offensive base formation in American football which requires only one running back lined up about five yards behind the quarterback. There are many variations on single back formations including two tight ends and two wide receivers, one tight end/three wide receivers, etc. The running back can line up directly behind the quarterback or offset either the weak side or the strong side.
In American football, the specific role that a player takes on the field is referred to as their "position". Under the modern rules of American football, both teams are allowed 11 players on the field at one time and have "unlimited free substitutions", meaning that they may change any number of players during any "dead ball" situation. This has resulted in the development of three task-specific "platoons" of players within any single team: the offense, the defense, and the so-called 'special teams'. Within these three separate "platoons", various positions exist depending on the jobs that the players are doing.
There are several defensive formations commonly used in eight-man football. Defensive formations are classified by the total number of linemen and linebackers in the formation. The three basic types of formations in eight-man football are seven-man fronts, six-man fronts and five-man fronts.
The triple option is an American football play used to offer several ways to move the football forward on the field of play. The triple option is based on the option run, but uses three players who might run with the ball instead of the two used in a standard option run.
In American football, a play is a close-to-the-ground plan of action or strategy used to move the ball down the field. A play begins at either the snap from the center or at kickoff. Most commonly, plays occur at the snap during a down. These plays range from basic to very intricate. Football players keep a record of these plays in a playbook.
In American football, the 3–4 defense is a common defensive alignment consisting of three down linemen and four linebackers. It is a called a "base defense" because it is the default defensive alignment used on "base downs". However, defenses will readily switch to other defensive alignments as circumstances change. Alternatively, some defenses use a 4–3 defense.
In American football, a 4–3 defense is a defensive alignment consisting of four down linemen and three linebackers. It is called a "base defense" because it is the default defensive alignment used on "base downs". However, defenses will readily switch to other defensive alignments as circumstances change. Alternatively, some defenses use a 3–4 defense.
The following terms are used in American football, both conventional and indoor. Some of these terms are also in use in Canadian football; for a list of terms unique to that code, see Glossary of Canadian football.
A defensive tackle (DT) is a position in American football that will typically line up on the line of scrimmage, opposite one of the offensive guards; however, he may also line up opposite one of the tackles. Defensive tackles are typically the largest and strongest of the defensive players. Depending on a team's individual defensive scheme, a defensive tackle may be called upon to fill several different roles. These roles may include merely holding the point of attack by refusing to be moved, or penetrating a certain gap between offensive linemen to break up a play in the opponent's backfield. If a defensive tackle reads a pass play, his primary responsibility is to pursue the quarterback, or simply knock the pass down at the line if it is within arm's reach. Other responsibilities of the defensive tackle may be to pursue the screen pass or drop into coverage in a zone blitz scheme. In a traditional 4–3 defense, there is no nose tackle. Instead there is a left and right defensive tackle. Some teams, especially in the National Football League (NFL), have a nose tackle in this scheme, but most of them do not.