The object of American football is to score more points than the opposing team within the time limit.
Field and players
American football is played on a field 360 by 160 feet (109.7 m × 48.8 m). 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 (91.4 m) apart. A scoring area called an end zone extends 10 yards (9.1 m) beyond each goal line to each end line. While the playing field is effectively flat, it is common for a field to be built with a slight crown—with the middle of the field higher than the sides—to allow water to drain from the field.
Yard lines cross the field every 5 yards (4.6 m), and are numbered every 10 yards from each goal line to the 50-yard line, or midfield (similar to a typical rugby league field). Two rows of short lines, known as inbounds lines or hash marks, run at 1-yard (91.4 cm) intervals parallel to the sidelines 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 goalposts (also called uprights) connected by a crossbar 10 feet (3.05 m) from the ground. For high skill levels, the posts are 222 inches (5.64 m) apart. For lower skill levels, these are widened to 280 inches (7.11 m).
Each team has 11 players on the field at a time. However, teams may substitute for any or all of their players, if time allows, during the break between plays. As a result, players have very specialized roles, and, sometimes (although rarely) almost all of the (at least) 46 active players on an NFL team will play in any given game. Thus, teams are divided into three separate units: the offense, the defense and the special teams.
