American football is played between two teams of up to 53 players (NFL rules), with each team sending 11 players onto the field of play, and with the ability to replace some or all of the players on the field at any time.
The team that gets possession of the ball is the offense, and the goal is to advance the ball as far as possible toward the opposing team's position in an effort to cross the goal line into the opposing team's end zone to score a touchdown. The offense can do this in two ways, by running the ball forward (Rushing) or by throwing the ball forward (Passing). The goal of the defense is to prevent the offense from scoring as much as possible and to force a loss of possession. If the offense scores or loses possession, the teams switch offenses and the game continues in this manner until the end of the four quarters. Early American football was essentially the same ball used in English rugby, but it was reduced in size and shaped in the late 19th century. The American rugby ball in use today is a long ellipsoid (prolate spheroid) about 11 to 11.25 inches (27.94 to 28.58 cm) long, with a circumference of about 21 to 21.25 inches (53.34 to 53.98 cm) around the center of the sphere, and weighing about 14 to 15 ounces (396.89 to 425.24 g). Although the sphere is olive-shaped, it is not closer to the standard oval shape as in the English rugby ball, but rather has slightly pointed ends in the form of a "vesica piscis" (meaning oval with two pointed ends), which is the most efficient shape for a long forward throw with one hand. The similar Canadian style rugby ball (also known as the Canadian soccer) is closer in appearance to the English rugby ball, and is slightly different in size than the American football, but the balls used in both sports are for the most part interchangeable. Each rugby ball consists of four pieces of leather (or plastic), each of which undergoes a series of quality checks for weight and cracks before workers begin the production process. Two of the pieces are perforated at the joints and tied together with string. One of the bound leather pieces is then pierced with two rows of holes and a reinforced center to hold the inflatable piston. Some or all of the structural leather pieces of the ball are embossed with the manufacturer's brand, game or league name, signature or other imprint. Each leather block is lined, and the four pieces of leather are sewn together in an inverted fashion without stitching where there are holes for tie-down cords, using the slit to turn the leather shell inside out, and finally a polyurethane or rubber bulb weighing approximately 12.5 to 13.5 ounces (354.37 to 382.72 grams) is slipped through the slit, and thick cords made from polyvinyl chloride or strips of leather tied along the perforations to tighten the slit. The ball needs to be inflated to an air pressure of 12.5 to 13.5 psi (86.2 to 93.1 kPa) before use.
The shells of balls used in NFL professional or NCAA college games must be made of leather and dyed the brown color of natural leather, while amateur recreational play balls are sometimes made of rubber or plastic. Modern tanning techniques allow the leather to be pebbled to increase friction so that the ball will not slip in dry or wet conditions. American football balls are usually painted with thick white semi-circular (two pieces of leather on either side of the stitching) stripes on both ends of the ball to increase visibility in low-light conditions, but NFL balls do not have white stripes. Early American footballs were also known as "pigskin" because they were made from inflated pig bladders.
According to NFL rules, home teams are required to have 36 pucks for outdoor games and 24 pucks for indoor games ready for play two hours before kickoff, which is measured by a barometer to comply with league regulations. Twelve brand new soccer balls, sealed in special boxes shipped by the manufacturer, are unsealed by the referee in the locker room two hours prior to the start of the game. These footballs are specially stamped with the letter "k" and are used exclusively for kicking games. American football stadiums are often nicknamed "Gridiron" because of the way the lines running across the field make it look like a wire rack that helps keep food on the grill. For this reason, American football and its Canadian counterpart are collectively known as gridiron soccer. Other English-speaking countries have even shortened the name to "gridiron" to refer to American football because it is more representative of the two. The standard field used by the NFL and NCAA is a 360-foot-long (120 yards or 109.7 m) by 160-foot-wide (53.33 yards or 48.8 m) rectangular turf field (some indoor games use faux-grass carpet), with a longer border called the sideline and a shorter border called the end line. End line in front of the marking line is called the score line (goal line), each side of the field between the end line and the score line has a depth of 10 yards (9.1 m) scoring area is called the end zone (end zone, also known as the touchdown zone), the corners of the end zone each have a about 1 foot long orange rectangular standard post (pylon). The scoring lines are 100 yards (91.44 meters) apart, and the area between them is the playing field. A yard line, or 5-yard line, is marked every 5 yards (4.6 meters) from the goal line, with numbers marked every 10 yards until the 50-yard line reaches midfield. In the middle of the field and both sides of the sideline and parallel to the side lines with horizontal short marking lines, called yard marks (hash marks, or whole yard line), which is close to the sideline of the yard marking line is called inbounds line (inbounds line). Any player must serve the ball on or between the yardage lines.
Under NFL rules, the inbounds line excludes the sideline and end zone themselves, and the ball is ruled out of bounds if it is on top of the sideline or end zone. The scoring line counts as part of the end zone, and an offense scores a touchdown with the ball in its possession if the ball is over the scoring line.
Two goalposts (also known as uprights), spaced 18.5 feet (5.64 m) apart, are located in the center of the end zone, connected by a crossbar 10 feet (3.048 m) above the ground. To be considered a successful shot, the ball must pass between the two goalposts and over the bar. At some lower levels of the game, the goal posts are sometimes widened to 23.33 feet (7.11 m) to increase the success of the shot. American football is an intensely competitive contact sport. Due to the violent rushes common in the game, players are required to wear armor-like protective gear. These include helmets with caged visors and upper body armor with integrated shoulder and chest pads, as well as mouth guards, gloves, and pads covering the elbows, lower back, hips, crotch, thighs, and knees depending on the level of the league.
Statistically, the use of protective gear can reduce the likelihood of serious injuries of all kinds by more than half. Nonetheless, the intense nature of American football still makes injuries unavoidable. Concussions, caused by the blunt force of a limb impact, are particularly common, and tens of thousands of players are injured each year. The standard American football game is divided into four 15-minute quarters, with the first two quarters being called the first half, and the third and fourth quarters being called the second half, with a 12-minute (20-minute in college games) halftime break between the first and second halves. The second half will begin with a restart just as the game began. There will be a two-minute break at the end of the first quarter of the first half and a two-minute break at the end of the third quarter of the second half, when the players will exchange the field, and the team that originally had possession of the ball will continue the offense after the exchange of the field.
The offense must restart its next possession within 40 seconds of the end of each possession or be penalized. In addition to the exchange of possession, the game clock is stopped in the following situations: a failed pass, a player going out of bounds with the ball, the referee blowing the whistle to penalize a foul, and the coach requesting a timeout (only three times). The game clock begins to continue to keep time when the referee re-places the ball at the kickoff for both teams to begin their offense.
Overtime
If the teams are tied at the end of four quarters of normal play, a 15-minute overtime period will be played after a three-minute break, with each team allowed one possession and two timeouts. At the start of overtime, the teams flip a coin to guess first, and the correct team gets to choose whether to play offense or defense first. Normally in the NFL regular season if the score is still tied after the first overtime period, the game ends in a tie. The playoffs, on the other hand, because they are elimination games that must have a winner, will be decided by the sudden death method, with the team that scores first winning the game. One drawback to this rule is that the team that wins the coin-guessing often chooses to get possession of the ball on offense and then take overtime on a field goal after advancing the line closer, leaving the other team with almost no chance at all of getting possession to recover the score. In the 2010 season, the NFL changed the rules so that either team had to score a touchdown to cause sudden death, and the other team could still get possession and a chance to get back in the game if it scored on a field goal.
College and high school games use the Kansas playoff rule, which means that no matter how the first offense scores, the other team still gets a turn in possession to get back in the game. If the game is still tied after the first overtime period, a second period is added, and if the game is still tied after the second period, a third period is added ...... until the game produces a winner. If the game is tied after the third quarter, the team that scores a touchdown will need to make a 2-point conversion to get the extra point (see "Scoring Methods" below). The game begins with a kickoff. The defense's kickoff man places the ball in the seats at the 30-yard line and kicks it to the offense. The offensive returner catches the ball and will advance it as far as possible until it is blocked by the defense. The offense then starts a drive from the point where the ball was blocked.
If the returner catches the kickoff in his own end zone, he has two choices: he can either carry the ball out of the end zone and run a line of scrimmage to the opposing team's field of play until he is intercepted, or he can carry the ball and then touch the ball with his knee to the ground to indicate a touchback, which signals a stoppage of play, and the ball will be placed on the attacking team's 20-yard line for the attacking team to start the first play of the drive. American football is played in a round-robin format, with offenses similar to those of six-a-side and union football. The offense has four chances to advance 10 yards to the front (the defense's end zone), each of which is called a "down" drive (i.e., a chance to be stopped and brought down once). When the offense succeeds in advancing 10 yards or more on fourth down, a new fourth down opportunity is awarded - called a new "first down" (1st down). By continually gaining new first downs, the offense can run consecutive series of downs moving forward until a score is scored. And the defense's goal is simple - it's to prevent the other team from advancing as far as possible within four downs to force an exchange of possession.
If the offense fails to reach its tactical goal of accumulating 10 yards of total offense on all four downs, it hands off possession to the opponent at the end of the fourth down. Typically, if the offense is still unsure of advancing a sufficient distance when the fourth down is initiated, a punt is used to move the ball to the opponent.
Except on the opening kickoff, at the half, and after a score when the kickoff is resumed, every drive begins with the same snap: the offense and defense line up on opposite sides of the line of scrimmage, facing each other. The center of the offense reaches into the neutral zone with one hand to hold the ball, then passes the ball backward between his legs to a teammate. This receiving teammate is usually the quarterback of the offense, but in special circumstances it may be one of the remaining teammates (such as a special teams ball handler). Neither the offense nor the defense may cross the offensive line before the center serves the ball, or a penalty will be called. From the moment the ball is served, the game is officially restarted and the teams continue to battle for possession and scoring opportunities.
There are two main ways for an offense to move forward:
1. The quarterback receives the ball from the center and either carries the ball himself or passes the ball to another teammate (usually a running back), who then runs forward with the ball in his hands, known as a rushing attack (rushing, or running attack).
2. The quarterback throws the ball a certain distance away to any one of his legal receivers (eligible receiver), called a pass (passing). Forward pass (forward pass) is a feature of American football, the offensive side can only pass forward once in each offense, and must be in the offensive line behind the position before allowed to throw the ball forward. The long forward pass is often referred to as the "Hail Mary pass" because it is so difficult and easy to intercept, and the success rate is so low that the passer prays for divine help.
There is no limit to the number and location of lateral and backward passes, but there is a high chance of being sacked on offense, so the lateral or backward pass is rarely seen in American football, and it is an extremely difficult skill.
There are several ways to lose a try:
1. The ball carrier is tackled by the defense and falls to the ground, resulting in a down by contact, or has not yet fallen to the ground, but the referee believes that the attack has been intercepted by the opposing team (tackle).
2. A pass thrown forward by the quarterback that goes out of bounds or touches the ground before anyone can catch it is called an incomplete pass. After an incomplete pass, the ball is returned to the original line of scrimmage, the offense uses up a first down, and the play is restarted.
3. A player with the ball goes out of bounds with the ball.
Additionally, if either team scores, or if a player's helmet falls off, the offense's chance is over, called a dead ball. The referee will blow his whistle to notify.
Players and spectators often need to know what the first down is and how many yards are left before they get four more chances. For example, before a first down, the TV or scoreboard will say, "1st & 10" (10 yards on 1st down), meaning "This is a first down, and the goal is to move forward 10 yards (in order to successfully accumulate 10 yards)". If a play moves forward 3 yards, it becomes: "2nd & 7" (2nd down 7 yards), meaning "second down, the goal is to move forward 7 yards" ...... etc... etc. If the offense is within 10 yards of the line of scrimmage, instead of reporting the number of yards needed, the offense reports "1st & Goal" for the first try, etc. The offense may retain possession all the way down the field. The offense retains possession of the ball until:
1. The offense fails to gain a new first down -- in other words, fails to advance ten yards in four plays. The defense may begin a counterattack where the last down was completed. This type of offense-defense exchange is called a stall-count exchange (turnover on downs).
2. The offense scores a touchdown, or a field goal. The team that scored on the original offense then sends in a special teams unit to kick off to the opposing team.
3. The offense punt (punt). Usually, if the attacking team fails to advance ten yards in the first three blocks, and the position exceeds the distance at which a field goal can be made, in order to avoid giving the opposing team a chance to start the attack at that place after the exchange of offense and defense, the punt will be used in the fourth blocking big kick.
4, the defensive player in the air to receive the offensive side of the pass, called the interception (interception). A defensive player who intercepts the ball may immediately move toward the opposing team's end zone until he is tackled, out of bounds, or scores a touchdown. If he is not touched by any of the original offensive players and a part of his body other than his hands or feet touches the ground, he does not fall on contact and may continue to run until he is tackled or touched by an opponent and falls to the ground. The same rule applies to receivers on the offensive side of the ball.
5. An offensive player drops the ball to the ground, i.e., fumbles it, and a defensive player gets to it first. As with an interception, the defense can counterattack in real time until it is tackled, goes out of bounds, or scores a touchdown.
* Offense turnovers and interceptions are collectively known as turnovers.
6. The attacking team fails in its field goal attempt. The defending team may start the offense at the last kickoff (in the NFL, the field goal). If the field goal is within twenty yards of the front of the end zone, the original defending team starts the drive at the twenty yard line.
7. It is less common for an offensive team to be tackled, out of bounds, or fouled in their own end zone, called a Safety.
If any of the above occurs, the offense and defense change sides - the offense loses possession of the ball and becomes the defense, and the defense becomes the offense. The offense and defense score points in the following ways:
1. Touchdown (abbreviated TD) for six points. When a player runs into the opponent's end zone with the ball, or receives a pass in the opponent's end zone (provided that there is contact with the ground when the ball is caught, and that the ball is not dropped or pushed out of the end zone by a tackle before it hits the ground), it is a touchdown. The original meaning of the English word "touchdown" is "touchdown", which originated from English rugby (the ball must touch the ground when it is in the scoring area in order to score a try), but there is no "must touch the ground" rule in American football. However, American football does not have a "must touch the ground" rule, as long as the ball crosses the top of the end zone while in possession. In the NFL, a touchdown is scored when any part of the ball, no matter how much, enters the end zone. In addition, contact of the ball with any of the posts in the corners of the end zone is also considered a touchdown, provided the ball carrier is not tackled to the ground prior to the contact.
After a touchdown, the scoring team is awarded an extra point, or conversion, try. The ball is placed on the opponent's 3-yard line (2-yard line in the NFL). The scoring team has the option of kicking the ball into the goal for an extra point, called a point after touchdown (PAT), or running, passing, etc. for another touchdown for a 2-point conversion. In the college and professional game, most of them add 1 point for a field goal. The success rate for extra point field goals is 94% in the pros and 93.8% in college, while the success rate for extra points is only about 43%. If the defense succeeds in stopping a plus-2, and gets the ball in its hands and successfully counters with a touchdown, two points are awarded (does not apply to the NFL).
2. Field goal (abbreviated FG), 3 points. A field goal is scored by kicking the ball over the crossbar of the goal and between the two goalposts, and if the ball bounces back off the goalposts it is ruled a failed field goal. The ball must first be placed on the ground, usually by the Holder, who is responsible for holding the ball upright on the ground, and then by the Placekicker, who shoots the ball, known as a placed kick. Another very uncommon method is for the kicker to drop the ball with a normal straight kick, but with a slightly delayed kick, just before the ball touches the ground and bounces, called a drop kick. A field goal is usually chosen when the offense is relatively close to the goal line but is having trouble advancing enough yards before the end of the fourth down, or when time is running out to score in another way. In high school games, a failed field goal results in a loss of possession to the defense, but in college games, if it's not a fourth-down play, the offense can continue to maintain possession of the ball.
3. Safety, commonly known as the "suicide kick," scores two points. When an offense is forced back into its own end zone and is tackled or drops the ball, the defense scores two points. Certain fouls committed by the offensive team in their own end zone will also result in a defensive point being awarded to the defense. When there is a rule violation or a disturbance of the fair order of play, the referee throws a small yellow flag near the spot of the foul. Most penalties are called by moving the ball a certain number of yards toward the offending team's end zone, and if the defense is penalized for backing up enough yards to give the offense the yardage it needs, the offense is automatically awarded a new first down. After the play has been stopped, the team that did not commit the foul has the option of accepting the penalty or accepting the result of the play at that time. For example, if the defense is offsides or moving early, but the offense manages to gain 8 yards on the play, the offense may choose to accept the penalty, which is a gain of 5 yards and a new first down (5 more yards), or accept the result of the play and waive the penalty and go on to the next play (2 more yards).
Common fouls are:
1. FALSE START: The offense has someone moving in front of the serve other than a player moving parallel behind the line of scrimmage. Penalty: 5 yards back.
2. OFFSIDE: A player crosses the ball before the serve, e.g., contacting an opponent, crossing into the neutral zone. Penalty: 5 yards back.
3, pulling (holding): the unfair use of pulling jersey, hooking or shoveling to prevent the opposing player normal action. The penalty is 10 yards if the offense commits a foul or happens to convert on offense or defense, and 5 yards and an automatic new first down if the defense commits a foul.
4. Pass interference: When the ball is passed, the defensive player pushes, hooks, pulls or knocks down the offensive receiver; or the offensive receiver uses the same method to deal with the defensive player in order to avoid the other team getting the ball. If the defense commits a foul, the offense has the option of accepting the foul or declining it; if the offense chooses to accept it, it advances fifteen yards from the kickoff line and automatically regains four first downs; if the offense declines it (usually when the offense succeeds in advancing more than fifteen yards beyond the yardage penalized on the previous play), it may also retake the kickoff from the spot of the foul and likewise gain four new first downs. If the offense is fouled, back off five yards. If a player commits a similar offense before the pass is thrown, a "pulling" or "illegal contact" penalty will be assessed.
5, personal foul (personal foul): cause package of six items: roughing the passer, roughing the kicker, roughing the player, pulling the opponent's facemask, stooping to charge the opponent with the top of the helmet, in the blocking of the opponent's field goal, the animal intention to jump and press the opponent (but if the defender is already at their own yard line will not be counted in the foul). The penalized team backs off 15 yards, and if the foul is committed by the defense the opposing team advances 15 yards and gets an automatic first down.
6. DELAY OF GAME: The offense fails to serve the ball when the 40-second turnover timer reaches zero. Penalty quarterback, 5 yards back.
7, illegal forward pass (illegal forward pass): the offense passes the ball forward from a position in front of the offensive line. Back up 5 yards and consume a first down.
8, illegal formation (illegal formation): the offensive side of the offensive line less than seven people in formation.
9, too many men on the field (too many men on the field): either side in the starting more than 11 people on the field will be penalized, back five yards. (Note: It is legal for either team to have fewer than 10 men on the field at the time of the encounter. For example, in the 2007 season the Washington Redskins, in honor of linebacker Sean Taylor, who was robbed and killed in his home, purposely lined up the defense with ten men on the road against the Buffalo Bills the week after his death.)
10, illegal change of formation (illegal shift): players on the offensive side of the exchange can change the position before the ball, but only listed in the offensive and defensive line behind the players, and only one person each time, the offensive side of the teammates to stop and then you can be easy, other players or illegal players will change the position of the players will be punished by 5 yards. The defense is exempt from this rule.
11. Illegal man downfield: The offense's legal receivers are the two players standing on the outside of the line of scrimmage and all the guards (players behind the line of scrimmage); any player other than those mentioned above who crosses the line of scrimmage during the passing attack is considered to be in violation of the law and is penalized by a 5-yard setback.
12. Lack of sportsmanship (unsportsmanlike conduct): Penalties can be imposed for a variety of reasons and will be determined by the referee. Common ones are: intentionally hitting the referee, verbally abusing the referee, intentionally prolonging the celebration after scoring a touchdown, using consecutive timeouts to freeze the opposing kicker (the defending team calls two consecutive timeouts before the kicker kicks the ball in order to freeze the opposing team and increase the psychological pressure), and so on. Penalty: 15-yard penalty setback.
If a foul is penalized for yards past the first down goal line, the offense is also awarded a new first down (e.g., if a defender is offside when the offense is on second-and-3, the offense is awarded a new first down). If both the offense and the defense are penalized for backing up on the yardage line in the end zone, the ball is placed at the 2-yard line to start the play; if the offense or defense commits a foul inside their own 2-yard line, no matter how many yards are penalized, the penalized yardage becomes half of the distance to the end zone until a portion of the ball is in the end zone, and the opponent receives a safety (in the 2009 Super Bowl, the Pittsburgh Steelers were penalized a safety in exactly the same situation). The use of the line of scrimmage (also known as the line of scrimmage) for turn-based contests is a hallmark of American football, and a key feature of its departure from the long, serpentine, juxtapositional advancement of the English style of rugby. The term "scrimmage" is derived from the word "scrum" (or scrummage) in English rugby, which is a play in which the forwards on both sides of the field of play line up in a group and push the opposing team backwards to compete for the ball. In American football, the line of scrimmage is actually two imaginary lines parallel to the scoring line, one cut in line with the front of the ball (defensive line of scrimmage) and the other cut in line with the back of the ball (offensive line of scrimmage), which are used to separate the players on both sides of the ball. The offense and defense line up for the ball on each side of the line of scrimmage.
The distance between the two lines, which is the length of the ball itself, creates a narrow virtual zone across the field called the neutral zone. No player is allowed to move into or through the neutral zone before an offense has been initiated, or he or she will be penalized for early movement or offsides -- except for the attacking center, who must reach into the neutral zone to touch the ball before it can be served.
The term "offensive line" is often used to refer to the one on the defense's side of the line of scrimmage, because the cumulative distance the offense needs to advance is based on the front of the ball. In addition to the five linemen, there must be at least two Offensive Ends lined up on the line of scrimmage (within one yard of the line of scrimmage is considered to be "on the line"), which is usually two of the trio of the tight end and the left/right wide receiver. Of the offensive linemen lined up on the offensive line, only the two outermost are considered to be eligible receivers; the rest are ineligible receivers who are not permitted to touch the quarterback's pass directly (unless the ball has already been touched by another eligible receiver), and who are not permitted to initiate forward passes in front of the offensive line. However, everyone on the defensive side of the ball, as well as any player beyond 1 yard behind the offensive line of scrimmage, is allowed to catch passes at will.
Modern video-broadcasting technology allows computers to virtually map the offensive and defensive lines (usually the blue line) on a screen so realistically that it looks as if it were actually drawn on the playing field. The scale that accumulates the distance the offense needs to advance ("x offense x yards") and the virtual goal line (usually the yellow line) can also be instantly mapped out to make it easier for TV viewers to understand the progress of the game.