A comparison of American football and rugby union is possible because of the games' shared origins, despite their dissimilarities.
Rules
Tackle
In the event of a tackle in rugby the player may pass the ball behind him provided he is not on the ground. If the tackled player is on the ground, the ball must be released, allowing any other player (including the opposition) to pick up the ball (usually a ruck forms) and the play continues. In the event of a tackle in American Football the play is concluded and the team on offense maintains possession usually if either the ball was advanced past the first down marker (resulting in a first down and a renewal of the down count) or else the down count advances (although if the play was the fourth down play and the ball is not advanced past the first down marker then the defensive team would gain possession).
Forward pass
In rugby union, it is against the rules to throw (pass) the ball in a forward direction (towards opponents in-goal area): a player in a position to receive such a pass would in most cases be offside anyway. In American football, a player behind the line of scrimmage (most often the quarterback) is permitted to throw the ball forward from behind the line of scrimmage, provided that only one forward pass may be attempted during each play. A player can attempt a forward pass if he has already received a backwards pass, provided he stays behind the line of scrimmage.
Composition of teams
Professional and most scholastic American football team play has evolved from a single team with all players except limited substitutions playing the entire game, to a specialized "platoon" system consisting of three separate units (offensive, defensive, and "special teams" used for kicking and punting) with only one of the three being on the field at a time. That is to say that in professional American football, the majority of players play in only one specialization (or "one side of the ball") – however, every player is eligible to play in any specialization.
In rugby the teams are divided into eight forwards and seven backs. Both groups of players participate in attacking and defensive plays and are on the pitch at the same time. Only the eight forwards take part in the "set pieces", which are ways to contest ball possession when there is a minor rule infringement or the ball passes out of bounds. These set pieces are scrums and line outs. A person's build and skill set determines which group they can play in. All forwards must be heavy and strong to scrummage well but not so heavy that they are too slow to partake in attacking plays. The backs are lighter and faster and include the expert kickers. The forwards numbered 1 to 8 are the players that need to have good all-round rugby skills as well as speed and strength. In professional rugby there are very few players who can play equally well in a variety of positions and most will play in the same position from youth. Every position in rugby has its own unique name (except for number 8) and associated skill base.
In most club and schoolhouse rugby union, the majority of players play both offense and defense, only being substituted for injury. Substitutes in American football can return to the game at any stoppage in play. In rugby union, any player substituted off for any reason, except for a 'blood' injury, is prohibited from returning to the field of play (with the possible exception of front-row forwards). Rugby teams may make up to seven substitutions. Each team must field 15 players at the start of each match. Players who are bleeding may receive medical treatment off pitch and are replaced until they have stopped bleeding. If the bleeding has not stopped after 15 minutes of "real time", the player will be permanently substituted. This can be down to officials' discretion, depending at which level the sport is played, where only minor medical assistance is needed to make the player fit to return to the field of play. Ejected players in American football can be replaced with a substitute.
In rugby union there are two different punishments. A yellow card can be shown for lesser infringements, which leads to the player being off the pitch for 10 minutes whilst the team will play with one less player for that duration. For more serious offenses, such as eye gouging or stamping in some cases, players can be shown a red card, which means they are off the pitch for the rest of the match and the team plays the rest of the match with 14 players. Players who are red carded automatically face a disciplinary hearing, and can be banned for a period of time from one week to permanently depending on the level of offence. Also, in a televised match, rugby union players can be "cited" (by an independent citing commissioner) for an act of foul play for which the referee did not send the player off. This means that players who committed an act of foul play which the referee did not see, or was more serious than the referee thought, still face a disciplinary panel and possible suspension.
In American football, players are only disqualified for Unsportsmanlike Conduct and related penalties (Fighting, Palpably Unfair Acts, etc.). Players sent off in this fashion must be replaced with another member of the team for the remainder of the game. In addition, disqualified players may be fined (in the NFL) and/or suspended (at all levels) for future games.
Duration of game A rugby union game is divided into two halves of 40 minutes (or shorter for lower-grade games, and breaks given halfway through each half if playing conditions are considered to be extreme) separated by a half time period of up to 15 minutes in an international match. Most notably, a rugby union game will continue after the scheduled end of a half (half-time or full-time) until the ball becomes dead - any occurrence that would have play restart with a scrum or lineout, or when a team scores. This has led to some "nail-biting" finishes where teams losing by only a small margin work their way towards scoring, and games can go on several minutes over time. Something similar can occur at the end of either half in American football, though usually for only a few seconds of additional time, because a play that begins before the half expires continues until the ball is dead regardless of whether there is any time left on the clock. In both sports, the clock is also stopped during substitutions and for injuries, so the referee does not need to add stoppage time as is done in soccer. American football games are made up of four quarters of 15 minutes each (less at the high school and youth levels), but the clock stops and starts according to specific rules, so that a 15-minute quarter lasts much longer.
In the televised version of American football (both professional and major college level), the duration of such stoppages is often extended to accommodate the airing of commercial advertisements; this does not occur outside of the televised environment, where breaks in play are comparable to those in rugby union. In addition to this, the half-time break is typically 12 to 15 minutes; this intermission allows for resetting of strategy in both rugby and American football and adjusting to the opponent's schemes. During the American football halftime period, entertainment is provided for the fans in attendance, ranging from marching band performances in high school and college games to big-name entertainment for the Super Bowl (which usually has a longer-than-normal halftime in order to set up and break down whatever stage the halftime performer(s) will use). The entertainment in rugby varies from club to club but often includes kicking competitions involving members of the crowd, or youth rugby teams playing quick tournament games. Teams in the Super Rugby competition in the Southern Hemisphere often have mascots; however, very few rugby union teams have cheerleaders.
Game play In both sports, the essence of the game is to carry the ball over the opponent's goal line (Rugby requires the ball to be placed on the ground with downward pressure to score). In both sports the ball may be passed backwards an unlimited number of times, but in American football the ball may be passed forward once (and only once) as long as the passer is behind the line of scrimmage, as opposed to rugby union, where the ball cannot be passed forward but only kicked or carried forward. Even when kicked, only the kicker or players behind the kicker are allowed to catch or interfere with play.
In both sports, play is stopped when the ball goes out of bounds, when a player or team commits a foul or after any scoring play. In American football, play is also stopped when a player is ruled down or when a forward pass falls incomplete.
The forward pass and the stoppage when a player with the ball is downed results in short plays and a generally episodic game play in American football, as opposed to the longer and more fluid passages of play found in rugby union. If a player in rugby is tackled then the ball must be released and any player arriving at the scene may pick up the ball and run with it. If two or more opposing players arrive at the same time then a ruck is formed and the players push each other to get at the ball before play continues.
In rugby, kicking during the flow of the game is done for tactical reasons (both offensive and defensive) or to score a goal. If the ball is recovered by the kicking team, it can lead to significant improvement in field position. It is also legal in rugby to kick at goal at any point in the game. This is called a drop goal. In American football, a team that kicks the ball during play automatically gives up possession and cannot recover the ball unless an error in catching the ball (drop), or while running after catching (fumble) is made by the receiving team; because of this, punting is typically done only when teams do not expect to be able to retain possession (i.e. on fourth down). Additionally, rule changes made in the late twentieth century mandated that field goals cannot be made in front of the line of scrimmage; this has led to the demise of the drop-kick field goal in American football. Catching (or recovering) and blocking kicked balls are integral skills in rugby as well as in American football.
Origins
Various forms of football have been played in Britain for centuries with different villages and schools having their own traditional rules.
The Football Association was formed in England in October 1863. Differences of opinion about the proposed laws led to the formation of the first governing body for rugby, the Rugby Football Union (RFU) in 1871. Laws were drawn up for rugby football which was now distinct from association football (soccer).
In 1872 rugby clubs were established in the San Francisco Bay Area, which were composed mainly of British expatriates. The first recorded rugby match in the United States occurred on May 14, 1874 between Harvard University in Cambridge, Massachusetts and McGill University of Montreal, Quebec when McGill challenged Harvard to a game using rules in place at the Montreal campus.
In 1876, Yale, Harvard, Princeton, and Columbia formed the Intercollegiate Football Association, a competition based on the traditional rules of rugby. The sport of American football evolved from these intercollegiate games (see History of American football).