By the middle of the 19th century football had split into two distinct entities. Still popular today, these two sports included the football association game, or soccer, and rugby, in which players ran with the ball and tackled. Modern football evolved out of these two sports. The football playing field is rectangular in shape, measuring 100 yd (91.4 m) long and 53.5 yd (48.9 m) wide. At both ends of the 100-yard dimension, white lines called goal lines mark off the entrances to the end zones. Each team defends one end zone. A team must carry, pass, or kick the ball into the 10-yd (9-m) end zone on the opponents' half of the field to score. Lines parallel to the end zones cross the field at 5-yd (4.5-m) intervals. These lines give the field a resemblance to a large gridiron. Another set of lines, known as the sidelines, runs along both sides of the field. In addition, two rows of lines, called hash marks, run parallel to the sidelines. The hash marks are 53 ft 4 in (16.3 m) from each sideline in college and high school football, and 70 ft 9 in (21.6 m) from each sideline in the National Football League(NFL). A game of football is divided into four periods, known as quarters, each consisting of 15 minutes of playing time. The first two periods constitute the first half; the second two comprise the second half. Between the halves, a rest period, usually lasting about 15 minutes, is permitted the players, who may leave the field. The teams change halves of the field at the end of each quarter. The clocks stop at the end of each quarter and at certain other times, when particular events occur or when designated by the officials.
Football is played by two opposing teams, each fielding 11 players. Each team tries to move the ball down the field to score in the end zone defended by its opponents.