Gameplay
American football is played between two teams of 11 players each, with an oval (specifically prolate spheroid[15]) ball.[2] The basic format of the game involves two teams, one of which is in possession of the ball. The team in possession of the ball, termed the offense, tries to advance the ball towards the end zone, the scoring area at either end of the field. The other team, termed the defense, tries to stop the offense and prevent them from advancing the ball.[16] A game begins with a kickoff; the kicking and receiving teams are determined by a pre-game coin toss. The kickoff occurs as a player on the kicking team (the placekicker) kicks the ball from a tee. The receiving team then catches the ball and runs towards the opposing end zone until they are tackled or step out of bounds, at which point the ball is considered dead. The point where the ball becomes dead is defined as the first line of scrimmage, and play begins from that point.
Advancement of the ball occurs as a series of up to four discrete plays or downs. The offense must gain at least 10 yards in the four downs to continue advancing the ball. Before the start of the play, an official places the ball on the field so that the long axis of the ball is parallel to the sidelines. It is placed on or between the two sets of hash marks along the line, parallel to the goal line, where the previous play was declared dead. The two teams line up on either side of the line of scrimmage or "neutral zone", which is defined as the space as wide as the length of the ball extending from sideline to sideline parallel to the goal lines. Neither team may cross this neutral zone before the start of the play. A play begins with the snap, which occurs when the ball is lifted from the ground by one player on the offense (the center) and passed backwards to a second player (usually the quarterback). Play then continues until the ball becomes dead, at which point the down is considered over. If the player was tackled, steps out of bounds or his forward progress was stopped, the ball is placed at the spot of the tackle or where the player stepped out of bounds. If the ball became dead from an incomplete forward pass, the ball is placed at the same place it was at the start of the prior down. If the play resulted in a first down, a new set of four downs are given to the offense. If the offense does not advance the ball 10 yards in the four plays, they lose control of the ball to the defensive team. Normally, teams attempt either a field goal or choose to punt the ball away to the other team on fourth down, depending on their position on the field, rather than risk losing control of the ball.[16] During the course of play, either team may be charged a penalty for violating various rules of play. An official will indicate that a penalty has occurred by throwing a yellow flag on the ground.[17]
Teams can score points in several ways in football; a touchdown, worth six points, is the primary means of scoring. It occurs when the offense advances the ball into the defense's end zone or when the defense takes control of the ball and, on the same play, advances the ball into the offense's end zone. Following a touchdown, the scoring team attempts to score either one or two points by kicking or advancing the ball into the end zone (known as the extra point and two-point conversion, respectively) from two (NFL) or three (amateur) yards back from the goal line. In practice, the extra point is virtually automatic at the professional level and slightly less so at the amateur level, while the two-point conversion has a significantly lower success rate.[18] Teams can also score by a field goal, which occurs when the offensive placekicks or drop kicks the ball over the cros*****ar and between the goal posts at the back of the end zone. The offense is awarded three points for a field goal. A safety is scored by the defense when they tackle the ballcarrier in their own end zone. Following an extra point/two-point conversion attempt or successful field goal the scoring team must kick the ball off, while the team that is scored upon must kick the ball off after a safety.[16]
