A set of rule changes drawn up from 1880 onward by Walter Camp, the "Father of American Football", established the snap, the line of scrimmage, eleven-player teams, and the concept of downs.

The sport is closely related to Canadian football, which evolved in parallel with and at the same time as the American game, although its rules were developed independently from those of Camp.

Rugby, like American football, is a sport in which two competing teams vie for control of a ball, which can be kicked through a set of goalposts or run into the opponent's goal area to score points.

[13] Princeton, Harvard, Yale, and Columbia then agreed to intercollegiate play using a form of rugby union rules with a modified scoring system.

[21] A 1905 peak of 19 fatalities nationwide resulted in a threat by President Theodore Roosevelt to abolish the game unless major changes were made.

[24] Other rule changes introduced that year included the reduction of playing time from 70 to 60 minutes and an increase of the distance required for a first down from 5 to 10 yards (4.6 to 9.1 m).

Despite these extra benefits, the game had a strict sense of amateurism at the time, and direct payment to players was frowned upon, if not prohibited outright.

Meanwhile, the AFL introduced many new features to professional football in the United States: official time was kept on a scoreboard clock rather than on a watch in the referee's pocket, as the NFL did; optional two-point conversions by pass or run after touchdowns; names on the jerseys of players; and several others, including expansion of the role of minority players, actively recruited by the league in contrast to the NFL.

Competition for players heated up in 1965, when the AFL New York Jets signed rookie Joe Namath to a then-record $437,000 contract (equivalent to $4.23 million in 2023).

The bidding war for players ended in 1966 when NFL owners approached the AFL regarding a merger, and the two leagues agreed on one that took full effect in 1970.

This agreement provided for a common draft that would take place each year, and it instituted an annual World Championship game to be played between the champions of each league.

[59] The leader of the offensive line is the center, who is responsible for snapping the ball to the quarterback, blocking,[59] and for making sure that the other linemen do their jobs during the play.

The main goal of the wide receiver is to catch passes thrown by the quarterback,[59] but they may also function as decoys or as blockers during running plays.

Turnovers include interceptions (a defender catching a forward pass intended for the offense) and forced fumbles (taking possession of the ball from the ball-carrier).

Their roles include defending the run, pressuring the quarterback, and tackling backs, wide receivers, and tight ends in the passing game.

[54] Three positions are specific to the field goal and PAT (point-after-touchdown) unit: the placekicker (K or PK), holder (H), and long snapper (LS).

Goal posts are padded at the base, and orange ribbons are normally placed at the tip of each upright as indicators of wind strength and direction.

[79] To contain the compressed air within it, a pig's bladder was commonly used before the advent of artificial rubber inside the leather outer shell to sustain crushing forces.

[80][81] At all levels of play, the football is inflated to 12+1⁄2 to 13+1⁄2 psi (86 to 93 kPa), or just under one atmosphere, and weighs 14 to 15 ounces (400 to 430 g);[78][82][83] beyond that, the exact dimensions vary slightly.

The visiting team can call either "heads" or "tails"; the winner of the toss chooses whether to receive or kick off the ball or which goal they wish to defend.

[124] Repeated concussions (and possibly sub-concussive head impacts[128]) can increase a person's risk in later life for CTE (chronic traumatic encephalopathy) and health issues such as dementia, Parkinson's disease, and depression.

Originally not intended as a rival league, the entry of owners who sought marquee talent and to challenge the NFL led to an escalation in salaries and ensuing financial losses.

[154] The United Football League (UFL) began in 2009 but folded after suspending its 2012 season amid declining interest and lack of major television coverage.

[156] American football leagues exist throughout the world, but the game has yet to achieve the international success and popularity of baseball and basketball.

[163][164] The continuing interest and growth in both the sport and the series has led to the possible formation of a potential NFL franchise in London[165][166][167] An American football league system already exists within the UK, the BAFANL, which has run under various guises since 1983.

The day on which the Super Bowl is held is considered a de facto national holiday,[185] and in parts of the country like Texas, the sport has been compared to a religion.

[196] Japan was introduced to the sport in 1934 by Paul Rusch, a teacher and Christian missionary who helped to establish football teams at three universities in Tokyo.

The NFL has expressed interest in having games in the country, and the Super Bowl has become a widely watched event in Brazil at bars and movie theaters.

[206][207] China has additionally been a target for the expansion of the sport, with the Mainland being the home of the Chinese National Football League as well as a growing audience of Super Bowl watchers.

[209] Although both games share a similar set of rules, there are several key rule differences: for example, in Canadian football the field measures 150 by 65 yards (137 by 59 m), including two 20-yard end zones (for a distance between goal lines of 110 yards),[210] teams have three downs instead of four, there are twelve players on each side instead of eleven,[211] fair catches are not allowed, and a rouge, worth a single point is scored if the offensive team kicks the ball out of the defense's end zone.