American football (known as football in the United States and gridiron in some other countries) is a sport played by two teams of eleven players on a rectangular field 120 yards long by 53.33 yards wide with goalposts at each end. The offense attempts to advance an oval ball (the football) down the field by running with or passing it. They must advance it at least ten yards in four downs to receive a new set of four downs and continue the drive; if not, they turn over the football to the opposing team. Points are scored by advancing the ball into the opposing team's end zone for a touchdown, kicking the ball through the opponent's goalposts for a field goal or by the defense tackling the ball carrier in the offense's end zone for a safety. The team with the most points at the end of a game wins.
American football evolved in the United States, originating from the sport of rugby football. The first game of American football was played on November 6, 1869 between two college teams, Rutgers and Princeton, under rules resembling rugby and soccer. A set of rule changes drawn up from 1880 onward by Walter Camp, the "Father of American Football", established the snap, eleven-player teams and the concept of downs, and later rule changes legalized the forward pass, created the neutral zone and specified the size and shape of the football.
In American football a play is a close to the ground "plan of action" or "strategy" used to move the ball down the field. A play occurs at either the snap from the center or at kickoff. Most commonly plays occur at the snap during a down. These plays vary between basic to very complicated. Football players keep a record of these plays in their playbook.
College football is American football played by teams of student athletes fielded by American universities, colleges, and military academies, or Canadian football played by teams of student athletes fielded by Canadian universities. It was through college football play that American football rules first gained popularity in the United States.
In American and Canadian football, a two-point conversion is a play a team attempts instead of kicking a one-point convert immediately after it scores a touchdown. In a two-point conversion attempt, the team that just scored must run a play from close to the opponent's goal line (5-yard line in Canadian, 3-yard line in amateur American, 2-yard line in professional American) and advance the ball across the goal line in the same manner as if they were scoring a touchdown. If the team succeeds, it earns two additional points on top of the six points for the touchdown. If the team fails, no additional points are scored. In either case, the team proceeds to a kickoff.
Various sources estimate the success rate of a two-point conversion to be between 40% and 55%, significantly lower than that of the extra point, though if the higher value is to be believed, a higher expected value is achieved through the two-point conversion than the extra point.
Goal refers to a method of scoring in many sports. It can also refer to the physical structure or area of the playing surface where scoring occurs.
In several sports, a goal is the sole method of scoring, and thus the final score is expressed in the total number of goals scored by each team. In other sports, a goal may be one of several scoring methods, and thus may be worth a different set number of points than the others. A few of these sports use the term field goal to distinguish one scoring method from another.