American Football 101
American football is played on a field 120 yards long by 53.3 yards. The longer boundary lines are sidelines, while the shorter boundary lines are end lines. Near each end of the field is a goal line, they are 100 yards apart. The scoring area called an end zone extends 10 yards beyond each goal line to each end line. Yard lines cross the field every 5 yards, and are numbered from each goal line to the 50-yard line, or midfield. Two rows of lines, known as inbounds lines or hash marks, parallel the side lines near the middle of the field. All plays start with the ball on or between the hash marks. At the back of each end zone are two goal posts (also called uprights) that are 18.5 feet apart. The posts are connected by a crossbar 10 feet from the ground.
              

To advancing the ball in American football the team that takes possession of the ball. The offense has four attempts, called downs, to advance the ball 10 yards towards their opponent's the defense's end zone. When the offense gains 10 yards they gets a first down, which means the team has another set of four downs to gain yet another 10 yards or score with. If the offense fails to gain a first down (10 yards) after 4 downs, the other team gets possession of the ball. Players can then advance the ball in two ways:


1. By running with the ball, also known as rushing. One ball-carrier can hand the ball to another player or throw backwards to another player. These are known as a handoff.


2. By throwing the ball to a teammate, known as a forward pass or as passing the football. The forward pass is a key factor distinguishing American football from other football sports. The offense can throw the ball forward only once on a play, only from behind the line of scrimmage and only before crossing the line of scrimmage.


The ball can be thrown, pitched, handed-off, or tossed sideways or backwards at any time. Once the player with the ball is forced to the ground (tackled) or has his forward progress halted by members of the other team the play is then over. If a forward pass flies out of bounds or touches the ground before it is caught. It is known as an incomplete pass. The ball is returned to the most recent line of scrimmage for the next down.

If the team fails to get a first down, they fail to move the ball past a line 10 yards ahead of where they got their last first down or loose yardage. The defensive team takes over the ball at the spot where the 4th-down play ends. A change of possession in this manner is commonly called a turnover on downs. If offense scores a touchdown or field goal. The team that scored then kicks the ball to the other team in a special play called a kickoff.

The offense punts the ball to the defense. A punt is a kick in which a player drops the ball and kicks it before it hits the ground. Punts are made on fourth down, when the offensive team does not want to risk giving up the ball to the other team at its current spot on the field (through a failed attempt to make a first down) and feels it is too far from the other team's goal posts to attempt a field goal.


A defensive player catches a forward pass. This is called an interception, and the player who makes the interception can run with the ball until he is tackled, forced out of bounds, or scores. An offensive player drops the ball (a fumble) and a defensive player picks it up. As with interceptions, a player "recovering" a fumble can run with the ball until tackled or forced out of bounds. Lateral passes that are not caught or caught by a defensive player are considered fumbles. Lost fumbles and interceptions are together known as turnovers.

A touchdown (TD) is worth 6 points. It is scored when a player runs the ball into or catches a pass in his opponent's end zone. After a touchdown, the scoring team attempts a conversion .The ball is placed at the other team's 3-yard line. The team can attempt to kick it over the crossbar and through the goal posts in the manner of a field goal for 1 point (an extra point or point after touchdown (PAT)), or run or pass it into the end zone in the manner of a touchdown for 2 points (a 2 point conversion).
A field goal (FG) is worth 3 points, and it is scored by kicking the ball over the crossbar and through the goal posts. A Safety worth 2 points and is scored by the defense when a ball-carrier is tackled in his own end zone. Safeties are also awarded if the offense fumbles the ball out-of-bounds in the end zone, has a kick blocked out of the end zone or commits certain penalties in the end zone. Safeties are relatively rare.

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 30-yard line. 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 series of 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 series from its own 20-yard line. A touchback also occurs when the kick goes out-of-bounds in the end zone. After safeties, the team that gave up the points kicks the ball to the other team from its own 20-yard line.
 

Variations


 Variations on these basic rules exist, particularly in flag football, which are designed as non-contact or limited-contact alternatives to the relative violence of regular American football. In flag football, tackling is not permitted. Offensive players are "tackled" when a defender tags them or removes a flag from their body, respectively.