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Questions about the sport of American football. This should not be confused with the [football] tag, which only applies to the sport called "soccer" in most of North America.

American football, simply called "football" in the United States (often conflicting with the international term for "association football" or "soccer"), is a game descended from rugby football. It is played by two teams of 11 players each (with substitutions from the sideline or "bench" allowed) on a field 360 feet long by 160 feet wide, comprised of a 100-yard "gridiron" with two "end zones" on opposite sides, separated from the main field of play by two "goal lines". The "football" is an oblong inflatable leather-skinned air bladder, originally similar to a rugby ball but now smaller and more pointed on each end to make it easier to throw in a stable spiral. The object for both teams is to score more points than their opponent by the end of regulation play (60 minutes of "clock time", divided into 15-minute "quarters").

Teams score points while in possession of the ball by carrying the ball across their opponents' goal line or completing a pass across that line into the "end zone" (a "touchdown", worth 6 points plus the opportunity to score an additional one or two points), or by kicking the ball through a goal post, composed of two uprights placed at the far end of the end zone 18 feet apart, with a horizontal crossbar 10 feet off the ground (a "field goal" worth 3 points).

One team begins the game in possession of the ball (usually decided by coin toss). That team is the "offense". They receive the ball during the "kickoff" (a place-kick by the opposing team), and move the ball as far up the field as possible before being "downed", either by being tackled to the ground by the other team or running out of bounds. The point on the field at which the player was ruled "down", or the furthest point of "forward progress" if a player was pushed back before being brought down, establishes a "line of scrimmage" parallel to the goal lines, from which the team begins a new play and again attempts to move the ball up the field toward the goal line.

The next plays are made "from scrimmage"; the offense lines up at the line of scrimmage, with the center lineman's hand on the ball and the quarterback directly behind him (sometimes a few yards back, in a formation known as "shotgun"). At a prearranged signal from the quarterback, the center will "snap" the ball backwards into the hands of the quarterback, who then decides what to do with it; he may throw or "pass" it forward to a "receiver" running downfield, he may "hand off" the ball to a "back" who will run with it, or in some circumstances the quarterback will keep the ball himself and run. Each time the player carrying the ball is "downed", or if a forward pass made during the play is not caught ("incomplete"), the team tries again on the next "down". The offense has four "downs" in which to either gain 10 yards (a "conversion" resulting in a new "first down"), or in any case, to score. On the "fifth down", the ball is given to the opposing team at the current line of scrimmage.

A team that is on their "fourth down" and not very close to getting a first down or the goal line will often elect to "punt"; the ball is snapped to a specialized player, the punter, who then drop-kicks it as far downfield as he can. This effectively gives the ball to the other team, who may catch the punt and try to carry it the opposite direction, but if well-executed, the opposing team will have much further to carry the ball for a scoring opportunity.

If the offense scores a touchdown or field goal, the ball is kicked off again by the scoring team to the defending team, who now becomes the offense. A rarer form of scoring is a "safety", which happens when the offense is downed behind their own goal line; the defending team gets two points, and the offense loses possession but gets a "free kick" to determine the opposing team's field position.

The team not in possession of the ball is called the "defense", and their object is to prevent the offense from scoring. They line up on the opposite side of the line of scrimmage from the offense, and when the ball is snapped they try to prevent the offense carrying the ball forward. They do this primarily by tackling the ball carrier (whoever that may be at any given moment), and by "covering" potential receivers as they move downfield, preventing them from successfully catching passes. The defense may also force a "turnover", by making a ball carrier drop the ball and then falling on it or picking it up themselves (a "fumble"), or by catching a pass in the air meant for a receiver (an "interception"). If they do either of these, the defense gains possession of the ball and becomes the offense.

Football is a "full-contact sport", similar to hockey and lacrosse and dissimilar to soccer and basketball, in which bodily collisions and relatively violent maneuvers are frequent and encouraged. As such, players wear protective pads and helmets with face cages, the exact specifications of which vary depending on the normal position of the player wearing them. A hard-shelled helmet with at least one metal loop forming a "facemask" (most modern helmets have a full metal "cage"), shoulder and chest pads, pants with tailbone, hip and thigh pads, a hard-shelled athletic "cup" for groin protection, and in some leagues knee and elbow pads, are required of all players. Despite this, injuries are common, ranging from muscle cramps and heat exhaustion from overexertion, to muscle and ligament injuries such as pulls, sprains and tears, to concussions and broken bones. Players have been paralyzed or even died playing the sport, though debilitating injuries and fatalities have dropped sharply in the professional leagues by a combination of better medical and protective technology, and by new rules of the game protecting players in certain situations shown to result in disproportionate injury.

The allowance in virtually all leagues of multiple player substitutions while play is stopped has led to a high amount of player specialization. Similar to other field or court sports like hockey or soccer, players typically specialize in a particular position on the field; however, unlike these sports where the players on the field must transition between offensive and defensive postures as the game progresses, football's focus on downs as discrete units of play has led to a specialization of offensive and defensive formations and variations. Because the entire 11-man side can be recalled and replaced, between each play if needed, the players on the team are generally only used for either offense or defense, and further, players specialize in a specific position, which may or may not be used in all formations of the offense or defense.

Common offensive roles include:

  • Quarterback - the primary play caller on the field and usually takes the direct snap to begin each play
  • Halfback or running back - primarily used to run the ball around or through the players at the line of scrimmage;
  • Fullback, similar to the running back but generally larger and slower, used for short gains
  • Offensive linesmen - further broken down into the "center" who snaps the ball to the quarterback, then tackles to either side of the center and guards outside the tackles; their primary role is to protect the quarterback and to make holes for the running back
  • Tight end - An "all-purpose" offensive player, this player lines up outside the tackle on one or both sides of the line of scrimmage, and can block or receive passes
  • Receivers - broken down into wide receivers and inside or "slot" receivers, these players attempt to run downfield and give the quarterback good opportunities to pass the ball to them

Common defensive roles include:

  • Defensive linesman - separated into defensive tackles on the inside and defensive ends outside, the defensive linesman's primary job is to try to prevent the ball getting past the line of scrimmage, by getting past the offensive linesmen to the ball carrier or by closing up holes that the ball carrier could use as "running lanes"
  • Linebacker - there are usually a total of seven combined linesmen and linebackers. What the linesmen are called depends on how many there are, where they're positioned and what they do after the snap; they are all-purpose defensive players who can come up to the line like linesmen to cover a run or get to the quarterback, or drop back into "pass coverage", guarding a man or an area of the field, typically toward the center and in short to medium range.
  • Safety - Usually at least one "free safety", sometimes an additional "strong safety" covering the wider portion of the field as divided by where the ball is placed, the safety's primary job is to prevent any offensive player getting behind him. Safeties generally cover passing routes far downfield, or serve as "backups" to the linebackers and cornerbacks.
  • Cornerback - Covers the wide receivers, or equivalently the areas of the field in medium range and along the edges of the field.

"Special teams" include players intended solely to kick the ball (the "kicker" and/or "punter") and to receive the ball (kickoff/punt returner; these are typically also wide receivers or running backs). In addition to their primary role, any offensive player is expected to block to create openings for the ball carrier, and any defensive player is encouraged to get their hands on the ball however they can.

A large number of rules infractions can be committed by either team, and the result of each play can sometimes be a matter of fractions of an inch. An officiating crew consisting of several referees wearing striped uniforms watches each play, deciding on the result of each play such as whether players were in bounds or out, or where the ball was located when the player carrying it was ruled down. They also look for illegal actions made by members of either team. Examples include procedural violations such as having too many men on the field or having them line up on the field in an illegal formation, not starting play in a timely manner or starting play before the ball was snapped, to gameplay violations like illegally restraining a player, physically blocking an eligible receiver, violently attacking players who do not have the ball or while the ball is not in play, or using a variety of illegal moves to block or tackle players.

Illegal actions result in punishment for the offending team in the form of a "penalty". A yellow flag is thrown by one or more officials to indicate a rule infraction seen by that official, and once play has stopped (or if it never started), the infraction is discussed among the referees and communicated to the teams and spectators by the head referee. A penalty typically consists of yardage; the line of scrimmage is moved forward or backward, to the detriment of the offending team. Other penalties include a "loss of down" or an "automatic first down" (usually, penalties result in the same down being re-played) and a "spot foul" (the ball is placed where the infraction occurred, usually because if the infraction had not occurred the play would have resulted in the same gain or loss of yardage). Rarely, players are ejected from the game for flagrant personal fouls resulting in serious injury, or for violence directed against the officiating staff or other non-players.

Relatively new to the game is "video review"; since virtually all NFL games and most NCAA games are recorded from many angles for TV, these leagues allow the officials to review the "instant replay" footage to ensure that the call they made was correct. Since the difference between a successful play and a failed one can be measured by fractions of an inch, and happen in a split second, these reviews generally reduce (but do not eliminate) the amount of criticism the officiating staff may receive for their handling of a game by fans, due to their ability to see these instant replays displayed on big screens in the stadium or on TV at home. Review requests can either come from officials on the field, officiating staff in the press box, or a "coach's challenge", indicated by a coach throwing a red flag onto the field. In order for a call made on the field to be reversed by review, it must first be a call that can be reversed (certain actions or inactions, such as a failure to call a penalty when one should have been, cannot be reviewed), and second, the video must show "indisputable evidence" from at least one viewing angle that the call was made incorrectly; evidence clearly proving the call was correct "confirms" the on-field ruling, while lack of such indisputable evidence either way means the on-field ruling "stands as called".

Football is played in the United States at virtually all age levels; municipal or local leagues have divisions for players as young as four years old, with public and private schools sponsoring teams beginning at the 6th to 7th grade and continuing through high school and college, to the adult professional level with the National Football League being the foremost professional league. NFL football is the most popular spectator sport in the United States.

Adult players who do not play in the NFL may choose to play at a recreational or semi-professional level in one of many local and regional leagues, or they may play in another professional league located outside the United States (such as the now-defunct European Football League, or the Canadian Football League which plays with significant changes to the field and to team composition) or a variant of football such as "arena football" (played in smaller indoor stadiums on a smaller field with fewer players). Typically, the number of players in leagues that do not present a likely path to the NFL is very small, as the risk of debilitating injury is very high for little reward.

Other variants of football include 6-man football, common in private and parochial schools and in smaller colleges that cannot field a competitive team for 11-man football. This variant is outwardly very similar to 11-man football, but with only 6 players per side, and an additional rule that the person who receives the snap must give the ball to someone else behind the line of scrimmage before it is allowed to cross the line of scrimmage.

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