Here are the basic rules of American football, explained for the novice. The rules described are those used in the professional NFL competition — college and high school football follow the same broad shape but differ in the detail, particularly in the timing and overtime rules.

Aim

  • The purpose of American football is to move the ball towards the opposition's end zone and ultimately into their end zone (a touchdown).
  • This is achieved by either running with the ball until tackled, or throwing the ball down field to a team mate.
  • Only one forward pass is allowed per play, and it must be thrown from behind the line of scrimmage. The ball may be passed backwards (a lateral) any number of times.

The Field

  • The field is 120 yards long and 53⅓ yards wide. The playing area between the two goal lines is 100 yards, with a 10-yard end zone at each end.
  • Lines are marked across the field every five yards, and the field is numbered every ten yards from each goal line, so the "50-yard line" is the halfway mark.
  • Goal posts stand at the back of each end zone, with the uprights 18 feet 6 inches apart and a crossbar 10 feet above the ground.

Teams

  • There are 11 players from each team on field at any one time.
  • NFL squads hold 53 players, with 48 dressed for each game. Substitution is unlimited between plays, so teams use entirely separate units for offense, defense and special teams (kicking plays).
  • The offensive unit is built around the quarterback, who receives the snap and either hands the ball to a running back or passes it to a wide receiver or tight end, all protected by five offensive linemen.
  • The defensive unit has a defensive line to stop the run and pressure the quarterback, linebackers behind them, and defensive backs (cornerbacks and safeties) covering the receivers.

Downs

  • Downs are the most fundamental part of NFL play. The offensive team moves the ball forward in sections of at least 10 yards. They have four chances (or downs) to gain those 10 yards. Each time the ball is advanced at least 10 yards within their four chances, another first down is earned, with four more chances to go a further 10 yards.
  • If the offensive team fails to move 10 yards within four downs, possession is surrendered at the spot where the ball was stopped — this is called a turnover on downs.
  • Because of that, the ball is usually punted (kicked away) to the defending team on fourth down, giving them the ball much further from the punting team's end zone. The alternative is to "go for it" — running or passing on fourth down as on any other down — which teams do when they need the yards badly enough to risk handing over good field position.
  • A field goal attempt is the other common fourth-down option when the team is close enough to kick.

Starting Play

  • Each half, and each restart after a score, begins with a kickoff. The kicker kicks from his own 35-yard line, while most of both teams line up near the receiving team's end of the field and may not move until the ball is caught or hits the ground.
  • If the ball is kicked into the end zone and downed there, the receiving team starts from its own 35-yard line (a touchback).
  • A team that is behind may instead attempt an onside kick — a deliberately short kick that it tries to recover itself.
  • Every other play starts with a snap: the ball is passed back between the centre's legs to the quarterback. The offense has 40 seconds between plays to snap the ball.

Timing

  • Four 15min quarters
  • Three time-outs per half for each team
  • 12min half-time
  • The clock stops for incomplete passes, when a player goes out of bounds, after a score and for time-outs, so a 60-minute game takes around three hours to play.

Overtime

  • If the scores are tied at the end of the fourth quarter, a 10-minute overtime period is played in the regular season, with each team having two time-outs.
  • Overtime is no longer sudden death. Both teams are guaranteed a possession, even if the team receiving first scores a touchdown. After each side has had the ball once, the next score wins.
  • If the scores are still level when the 10 minutes expire, the regular season game is recorded as a tie.
  • In the playoffs a game cannot end in a tie. Overtime periods of 15 minutes are played, as many as are needed, until a winner is decided.

Scoring

  • A touchdown is worth six points, and is scored when a team crosses the opposition's goal line with the ball, or catches or collects the ball in the end zone.
  • A field goal is worth three points. Field goals are usually attempted on fourth down if the kicker is close enough to the end zone to kick the ball through the posts.
  • An extra point is worth one point, earned by kicking the ball through the uprights after a touchdown. The ball is snapped from the 15-yard line, making it a kick of about 33 yards.
  • A two-point conversion is the alternative after a touchdown: instead of kicking, the team snaps from the 2-yard line and must take the ball into the end zone again by running or passing.
  • A safety is worth two points to the defensive team, awarded when a member of the offensive team is tackled with the ball in his own end zone. The team conceding the safety must then kick the ball back to the opposition.
  • The defensive team can also score two points by intercepting or recovering the ball during a conversion attempt and returning it the length of the field.

Turnovers

  • An interception is when a defensive player catches a forward pass. His team takes over possession immediately and may run with the ball.
  • A fumble is when a ball carrier drops the ball before he is down. The ball is live, and either team may recover it.
  • A turnover on downs is when the offense fails to make 10 yards in its four downs, as described above.

Common Penalties

Officials throw a yellow flag when a rule is broken, and the offending team is normally moved backwards (or the other team moved forwards) by a set number of yards. Some of the most common are:

  • False start — an offensive player moves before the snap. 5 yards.
  • Offside — a defensive player is across the line of scrimmage at the snap. 5 yards.
  • Delay of game — the ball is not snapped before the play clock expires. 5 yards.
  • Holding — grabbing an opponent who does not have the ball. 10 yards against the offense; 5 yards and an automatic first down against the defense.
  • Pass interference — illegally impeding a receiver trying to catch a pass. Against the defense the ball is placed at the spot of the foul, which can be a very large gain.
  • Face mask — grabbing the face mask of an opponent's helmet. 15 yards and an automatic first down.
  • Roughing the passer — hitting the quarterback after he has thrown. 15 yards and an automatic first down.
  • Intentional grounding — the quarterback throws the ball away under pressure with no receiver nearby. 10 yards or the spot of the foul, and loss of down.

How It Differs From Rugby

American football evolved from rugby in the United States during the late 19th century, but it is now a very different sport. The most important differences are the forward pass, which rugby does not allow; the system of downs, which gives the attacking team a set number of attempts to make ground; blocking, where players may legally obstruct opponents who do not have the ball; unlimited substitution, which allows completely separate offensive and defensive units; and the protective helmet and pads.