American Football American football , known in the United States as football , is a sport played between two teams of eleven . The objective of the game is to score points by advancing the ball into the opposing team ' s end zone . The ball can be advanced by running with it or throwing it to a teammate . Points can be scored by carrying the ball over the opponent ' s goal line , catching a pass thrown over that goal line , kicking the ball through the opponent ' s goal posts or tackling an opposing ball carrier in his own end zone . In the United States , high school football , college football and professional football are played under slightly different rules . High school football is governed by the National Federation of State High School Associations , college football by the National Collegiate Athletic Association and National Association of Intercollegiate Athletics while the major professional league is the NFL . The sport is also played in Europe , Japan and Mexico . The International Federation of American Football acts as an international governing body for the sport . American football is closely related to Canadian football but with some differences in rules and the field . History . The history of American football can be traced to early versions of rugby football and association football . Both games have their origins in varieties of football played in the United Kingdom in the mid - 19th century , in which a ball is kicked at a goal and / or run over a line . Many games known as " football " were being played at colleges and universities in the United States in the first half of the 19th century . American football resulted from several major divergences from rugby football , most notably the rule changes instituted by Walter Camp , considered the " Father of American Football " . Among these important changes were the introduction of the line of scrimmage and of down - and - distance rules . In the late 19th and early 20th centuries , game play developments by college coaches such as Eddie Cochems , Amos Alonzo Stagg , Knute Rockne , and Glenn " Pop " Warner helped take advantage of the newly introduced forward pass . The popularity of collegiate football grew as it became the dominant version of the sport for the first half of the twentieth century . Bowl game s , a college football tradition , attracted a national audience for collegiate teams . Bolstered by fierce rivalries , college football still holds widespread appeal in the US . The origin of professional football can be traced back to 1892 , with William " Pudge " Heffelfinger ' s $ 500 contract to play in a game for the Allegheny Athletic Association against the Pittsburgh Athletic Club . The first Professional " league " was the Ohio League , formed in 1903 , and the first Professional Football championship game was between the Buffalo Prospects and the Canton Bulldogs in 1919 . In 1920 , the American Professional Football Association was formed . The first game was played in Dayton , Ohio on October 3 , 1920 with the host Triangles defeating the Columbus Panhandles 14 – 0 . The league changed its name to the National Football League ( NFL ) two years later , and eventually became the major league of American football . Initially a sport of Midwestern industrial towns in the United States , professional football eventually became a national phenomenon . Football ' s increasing popularity is usually traced to the 1958 NFL Championship Game , a contest that has been dubbed the " Greatest Game Ever Played " . A rival league to the NFL , the American Football League ( AFL ) , began play in 1960 ; the pressure it put on the senior league led to a merger between the two leagues and the creation of the Super Bowl , which has become the most watched television event in the United States on an annual basis . Rules . Field and players . American football is played on a field . The longer boundary lines are " sidelines " , while the shorter boundary lines are " end lines " . Sidelines and end lines are out of bounds . Near each end of the field is a " goal line " ; they are apart . A scoring area called an " end zone " extends beyond each goal line to each end line . The end zone includes the goal line but not the end line . While the playing field is effectively flat , it is common for a field to be built with a slight crown — with the middle of the field higher than the sides — to allow water to drain from the field . " Yard lines " cross the field every , and are numbered every 10 yards from each goal line to the 50 - yard line , or midfield ( similar to a typical rugby league field ) . Two rows of short lines , known as inbounds lines or " hash marks " , run at 1 - yard ( 91 . 4 cm ) intervals perpendicular to the sidelines near the middle of the field . All plays start with the ball on or between the hash marks . Because of the arrangement of the lines , the field is occasionally referred to as a " gridiron " in a reference to the cooking grill with a similar pattern of lines . At the back of each end zone are two " goalpost s " ( also called " uprights " ) connected by a crossbar from the ground . For high skill levels , the posts are apart . For lower skill levels , these are widened to . Each team has 11 players on the field at a time . Usually there are many more players off the field . There are 53 active players on an NFL team . However , if time allows , teams may substitute for any or all of their players during the breaks between plays . As a result , players have very specialized roles and are divided into three separate units : the offense , the defense and the special teams . It is rare for all team members to participate in a given game , as some roles have little utility beyond that of an injury substitute . Start of halves . Similarly to association football , the game begins with a coin toss to determine which team will kick off to begin the game and which goal each team will defend . The options are presented again to start the second half ; the choices for the first half do not automatically determine the start of the second half . The referee conducts the coin toss with the captains ( or sometimes coaches ) of the opposing teams . The team that wins the coin toss has three options : Whatever the first team chooses , the second team has the option on the other choice ( for example , if the first team elects to receive at the start of the game , the second team can decide which goal to defend ) . At the start of the second half , the options to kick , receive , or choose a goal to defend are presented to the captains again . The team which did not choose first to start the first half ( or which deferred its privilege to choose first ) now gets first choice of options . Game duration . A standard football game consists of four 15 - minute quarters ( 12 - minute quarters in high - school football and often shorter at lower levels ) , with a 12 - minute half - time intermission after the second quarter . At all levels , a down ( play ) that begins before time expires is allowed to continue until its completion , even after the clock reaches zero . The clock is also stopped after certain plays , therefore , a game can last considerably longer ( often more than three hours in real time ) , and if a game is broadcast on television , TV timeouts are taken at certain intervals of the game to broadcast commercials outside of game action . If an NFL game is tied after four quarters , the teams play an additional period lasting up to 15 minutes . In a regular season NFL overtime game , the first team that scores wins , even if the other team does not get a possession ; this is referred to as sudden death . However , in a post - season NFL game during the playoffs , if the first team with possession scores only a field goal , the other team is allowed the opportunity to match or better this score . This rule only affects playoff games in overtime in which the first team with possession scores a field goal : if the first team with possession scores a touchdown , the sudden death rules take effect . In a regular - season NFL game , if neither team scores in overtime , the game is a tie . In an NFL playoff game , additional overtime periods are played , as needed , to determine a winner . College overtime rules are more complicated . Advancing the ball . Advancing the ball in American football resembles the " six - tackle rule " and the " play - the - ball " in rugby league . The team that takes possession of the ball ( the offense ) has four attempts , called downs , in which to advance the ball at least toward their opponent ' s ( the defense < nowiki > ' < / nowiki > s ) end zone . When the offense succeeds in gaining at least 10 yards , it gets a first down , meaning the team starts a new set of four downs to gain yet another 10 yards or to score . If the offense fails to gain a first down ( 10 yards ) after four downs , the other team gets possession of the ball at the point where the fourth down ended , beginning with their first down to advance the ball in the opposite direction . Except at the beginning of halves and after scores , the ball is always put into play by a snap . Offensive players line up facing defensive players at the line of scrimmage ( the position on the field where the play begins ) . One offensive player , the center , then passes ( or " snaps " ) the ball backwards between his legs to a teammate behind him , usually the quarterback . Players can then advance the ball in two ways : A down ends , and the ball becomes dead , after any of the following : Officials blow a whistle to notify players that the down is over . Before each down , each team chooses a play , or coordinated movements and actions , that the players should follow on a down . Sometimes , downs themselves are referred to as " plays . " Change of possession . The offense maintains possession of the ball unless one of the following things occurs : Scoring . A team scores points by the following plays : Kickoffs and free kicks . Each half begins with a kickoff . Teams also kick off after scoring touchdowns and field goals . The ball is kicked using a kicking tee from the team ' s own line in the NFL and college football ( as of the 2007 season ) . The other team ' s kick returner tries to catch the ball and advance it as far as possible . Where he is stopped is the point where the offense will begin its drive , or series of offensive plays . If the kick returner catches the ball in his own end zone , he can either run with the ball , or elect for a touchback by kneeling in the end zone , in which case the receiving team then starts its offensive drive from its own 20 yard line . A touchback also occurs when the kick goes out - of - bounds in the end zone . ( Punts and turnovers in the end zone can also result in a touchback ) . A kickoff that goes out - of - bounds anywhere other than the end zone before being touched by the receiving team is a foul , and the ball will be placed within the hash marks of the yard line where it went out of bounds , or 30 yards from the kickoff spot , depending on which is more advantageous to the receiving team . Unlike with punts , once a kickoff goes 10 yards and the ball has hit the ground , it can be recovered by the kicking team . A team , especially one who is losing , can try to take advantage of this by attempting an onside kick . After safeties , the team that gave up the points must free kick the ball to the other team from its own 20 yard line . Penalties . Fouls ( a type of rule violation ) are punished with penalties against the offending team . Most penalties result in moving the football towards the offending team ' s end zone . If the penalty would move the ball more than half the distance towards the offender ' s end zone , the penalty becomes half the distance to the goal instead of its normal value . Most penalties result in replaying the down . Some defensive penalties give the offense an automatic first down . Conversely , some offensive penalties result in loss of a down ( loss of the right to repeat the down ) . If a penalty gives the offensive team enough yardage to gain a first down , they get a first down , as usual . The only penalty that results in points is if a team on offense commits a certain fouls , such as holding , in its own end zone , which results in a safety . If a foul occurs during a down ( after the play has begun ) , the down is allowed to continue and an official throws a yellow penalty flag near the spot of the foul . When the down ends , the team that did not commit the foul has the option of accepting the penalty , or declining the penalty and accepting the result of the down . Players . Most football players have highly specialized roles . At the college and NFL levels , most play only offense or only defense . Offense . At least seven players must line up on the line of scrimmage on every offensive play . The other players may line up anywhere behind the line . The exact number of running backs , wide receivers and tight ends may differ on any given play . For example , if the team needs only one yard , it may use three tight ends , two running backs and no wide receivers . On the other hand , if it needs 20 yards , it may replace all of its running backs and tight ends with wide receivers . Defense . In contrast to members of the offense , the rules of professional football ( NFL Rulebook ) and American college football ( NCAA Rulebook ) do not specify starting position , movement , or coverage zones for members of the defensive team , except that they must be in the defensive zone at the start of play . The positions , movements and responsibilities of all defensive players are assigned by the team by selection of certain coverages , or patterns of placement and assignment of responsibilities . The positional roles are customary . These roles have varied over the history of American football . The following are customary defensive positions used in many coverages in modern American football . Special teams . The units of players who handle kicking plays are known as special teams . Three important special - teams players are the punter , who handles punts , the placekicker or kicker , who kicks off and attempts field goals and extra points , and the long snapper , who snaps the ball for extra points , field goals , and punts . Also included on special teams are the returners . These players return punts or kickoffs and try to get in good field position . These players can also score touchdowns . Uniform numbering . In the NFL , ranges of uniform numbers are ( usually ) reserved for certain positions : NCAA and high school rules specify only that offensive linemen must have numbers in the 50 – 79 range , but the NCAA " strongly recommends " that quarterbacks and running backs have numbers below 50 and wide receivers numbers above 79 . This helps officials , as it means that numbers 50 to 79 are ineligible receivers , or players that may not receive a forward pass ( except in the rare instance when a Tackle lines up as the outermost lineman on his side of the line and the officials are notified that he will be an eligible receiver for that particular play ) . There are no numbering restrictions on defensive players in the NCAA , other than that a team may not have two players on the field at the same time with the same jersey number . Basic strategy . Because the game stops after every down , giving teams a chance to call a new play , strategy plays a major role in football . Each team has a playbook of dozens to hundreds of plays . Ideally , each play is a scripted , strategically sound team - coordinated endeavor . Some plays are very safe ; they are likely to get only a few yards . Other plays have the potential for long gains but at a greater risk of a loss of yardage or a turnover . Generally speaking , rushing plays are less risky than passing plays . However , there are relatively safe passing plays and risky running plays . To deceive the other team , some passing plays are designed to resemble running plays and vice versa . These are referred to as play - action passes and draws , respectively . There are many trick or gadget plays , such as when a team lines up as if it intends to punt and then tries to run or pass for a first down . Such high - risk plays are a great thrill to the fans when they work . However , they can spell disaster if the opposing team realizes the deception and acts accordingly . The defense also plans plays in response to expectations of what the offense will do . For example , a " blitz " ( using linebackers or defensive backs to charge the quarterback ) is often attempted when the team on defense expects a pass . A blitz makes downfield passing more difficult but exposes the defense to big gains if the offensive line stems the rush . Many hours of preparation and strategizing , including film review by both players and coaches , go into the days between football games . This , along with the demanding physicality of football ( see below ) , is why teams typically play at most one game per week . Physicality . American football is a collision sport . To stop the offense from advancing the ball , the defense must tackle the player with the ball by knocking or pulling him down . As such , defensive players must use some form of physical contact to bring the ball - carrier to the ground , within certain rules and guidelines . Tacklers cannot kick or punch the runner . They also cannot grab the face mask of the runner ' s helmet or lead into a tackle with their own helmet ( " spearing " ) . Despite these and other rules regarding unnecessary roughness , most other forms of tackling are legal . Blockers and defenders trying to evade them also have wide leeway in trying to force their opponents out of the way . Quarterbacks are regularly hit by defenders coming on full speed from outside the quarterback ' s field of vision . This is commonly known as a blindside . To compensate for this , players must wear special protective equipment , such as a padded plastic helmet , shoulder pads , hip pads and knee pad s . These protective pads were introduced decades ago and have improved ever since to help minimize lasting injury to players . An unintended consequence of all the safety equipment has resulted in increasing levels of violence in the game . Players may now hurl themselves at one another at high speeds without a significant chance of injury . The injuries that do result tend to be severe and often season or career - ending and sometimes fatal . In previous years with less padding , tackling more closely resembled tackles in Rugby football . Better helmets have allowed players to use their helmets as weapons . This form of tackling is particularly unwise , because of the great potential for brain or spinal injury . All this has caused the various leagues , especially the NFL , to implement a complicated series of penalties for various types of contact . Most recently , virtually any contact with the helmet of a defensive player on the quarterback , or any contact to the quarterback ' s head , is now a foul . During the late 1970s , the penalty in high school football for spearing included ejection from the game . Despite protective equipment and rule changes to emphasize safety , injuries remain very common in football . It is increasingly rare , for example , for NFL quarterbacks or running backs ( who take the most direct hits ) to make it through an entire season without missing some time to injury . Additionally , 28 football players died from direct football injuries in the years 2000 - 05 and an additional 68 died indirectly from dehydration or other examples of " non - physical " dangers , according to the National Center for Catastrophic Sport Injury Research . Concussion s are common , with about 41 , 000 suffered every year among high school players according to the Brain Injury Association of Arizona . In 1981 , U . S . President Ronald Reagan , who played football in high school , commented on the contact of the sport : " Football is the last thing left in civilization where men can literally fling themselves bodily at one another in combat and not be at war . " Extra and optional equipment such as neck rolls , spider pads , rib protectors ( referred to as " flak jackets " ) , and elbow pad s help against injury as well , though they do not tend to be used by the majority of players due to their lack of requirement . The danger of football , and the equipment required to reduce it , make regulation football impractical for casual play . Flag football and touch football are less violent variants of the game popular among recreational players . Nutrition and dehydration . Football players typically begin their season while the weather is still extremely warm and with the dangerous combination of warm weather and high humidity , dehydration is a great risk for the players . The players are usually required to follow a hydration schedule . It is extremely important for players to drink enough fluids because dehydration can seriously reduce athletic performance and increase the risk of heat illnesses . Most trainers and coaches make it imperative for their players to drink fluids before they are thirsty . Brain injury . The Concussions Committee of the NFL , co - chaired by Dr . Ira Casson , has generally denied that concussions result in permanent brain injury . However , there is some research , reported in 2009 , which , using phone interviews based on the National Health Interview Survey , showed increased incidence of diagnosis of memory loss and dementia among retired professional football players . Such symptoms are believed related to the effects of concussion . More rigorous research is being conducted by Dr . Casson , neurologist , for the NFL . This finding is considered significant as such injuries may potentially affect high school and college players also . Organization in the United States . In the United States , the major forms are high school football , college football and professional football . Most American high school s field football teams . In general , high school teams play only against other teams within the same state , but there are some exceptions like nearby schools located on opposite sides of a state line . Most of college football in the United States is governed by the National Collegiate Athletic Association ( NCAA ) , and most colleges and universities around the country have football teams . These teams mostly play other similarly sized schools , through the NCAA ' s divisional system , which divides the schools into four divisions : Division I Bowl Subdivision , Division I Championship Subdivision , Division II , and Division III . Unlike the three smaller NCAA football divisions , the Division I Bowl Subdivision does not have an organized tournament to determine its national champion . Instead , teams are invited to compete in a number of post - season bowl games . In addition , the champions of six conferences in the Division I Bowl Subdivision receive automatic bids , and four other schools receive " at - large " bids , to those five bowl games under the highly lucrative Bowl Championship Series to help determine the national champion . The highest level major professional league in the United States is the 32 - team National Football League ( NFL ) . Another professional league , the 5 - team United Football League , also currently operates . Several semi - professional , women ' s semi - professional football , and indoor football leagues are also played across the country . Calendar . Traditionally , football is an autumn sport . A season typically begins in mid - to - late August and runs through December , into January . The NFL playoffs run further through January , and the Super Bowl is often played in the first week of February . As such , the sport is played in a wide range of weather conditions , from the heat and humidity of late summer , to the cold winds and snow in mid - winter . Unlike rainouts in baseball , play is only generally postponed or canceled in very extreme conditions such as lightning and hurricanes . The NFL draft is usually held in April , in which eligible college football players are selected by NFL teams , the order of selection determined by the teams ' final regular season records . It is a long - standing tradition in the United States ( though not universally observed ) that high school football games are played on Friday night , college games on Saturday , and professional games on Sunday . In the 1970s , the NFL began to schedule one game on Monday nights . Beginning in 2006 , the NFL began scheduling games on Thursday and Saturday nights after the college football regular season concludes in mid - November , aired on the NFL Network . Nationally televised Thursday - night college games have become a weekly fixture on ESPN , and most nights of the week feature at least one college game , though most games are still played on the traditional Saturday . Certain fall and winter holiday s — such as the NFL ' s Thanksgiving Classic and numerous New Year ' s Day college bowl game s — have traditional football games associated with them . Despite this , there are a few professional leagues that have played in the spring , mainly to avoid competition with the established leagues . Examples include the now defunct XFL , the United States Football League , and the proposed All American Football League . Indoor football is played primarily in spring for this same reason . At most levels of competition , college football teams hold several weeks of practices in the spring . These practices typically end with an intramural scrimmage open to the public . In certain areas , high school football teams also hold spring practices . Outside the United States . Outside the United States , the sport is referred to as " American football " ( or a translation thereof ) to differentiate it from other football codes such as association football ( soccer ) , rugby football , Australian rules football and Gaelic football . In Australia and New Zealand the game is also known as gridiron football , or more commonly as gridiron , although in the United States the term " gridiron " refers only to the playing field itself . The term " gridiron " has also been used in the UK to describe the game . In much of the world , the term football is unambiguous and refers to association football ( known commonly as ' soccer ' in the United States ) . Exportation by the established U . S . leagues and organizations . In 1977 , the semi - pro football teams Newton Nite Hawks and Chicago Lions completed a five - game tour in Europe . These were the first professional or semi - professional American football teams to play organized games on the European continent . Games were played in Versailles , France ; Lille , France ; Landstuhl , Germany ; Gratz Austria ; and Vienna , Austria . In 1985 , Bethany College head coach and future College Football Hall of Fame member Ted Kessinger brought the first American football team to play in Sweden . The Bethany " Terrible Swedes " defeated the Swedish all - star team 72 – 7 in Stockholm Olympic Stadium . The NFL has attempted to introduce the game to other nations and operated a developmental league , NFL Europa ( also known as the World League of American Football and NFL Europe ) with teams in various European cities , but this league was closed down following the 2007 season . The professional Canadian Football League and collegiate Canadian Interuniversity Sport play under the slightly different Canadian rules . Major American leagues have also held some regular season games outside of the United States . On October 2 , 2005 , the Arizona Cardinals and San Francisco 49ers played the first regular season NFL game outside of the United States , in Mexico City ' s Estadio Azteca , From 2007 , the NFL has played or has plans to play at least one regular season game outside of the United States during each season . The NCAA will also play games outside of the U . S . In 2012 , The United States Naval Academy will play the University of Notre Dame in Dublin , Ireland . The IFAF and the American Football World Cup . The International Federation of American Football ( IFAF ) is the " de facto " governing body for American football , with 45 member associations from North and South America , Europe , Asia and Oceania . The organization is headquartered in La Courneuve , France . Although the IFAF has relatively little standing in the U . S . compared to the NFL , NCAA , and the other established aforementioned bodies , these same organizations also give support to USA Football , the designated U . S . representative to the IFAF . The IFAF also oversees the American Football World Cup , which is held every four years . Japan won the first two World Cups , held in 1999 and 2003 . Team USA , which had not participated in the previous World Cups , won the title in 2007 . A long term goal of the IFAF is for American football to be accepted by the International Olympic Committee as an Olympic sport . The only time that the sport was played was at the 1932 Summer Olympics in Los Angeles , but as a demonstration sport .