• An NFL game has four 15-minute periods.Football official standing next to football image by Pezography from Fotolia.com
  
  Watching NFL games is a Sunday tradition for many Americans, and understanding the rules can make it even more enjoyable. The NFL Rule Book is published by the league and is made available to all players and coaches and fans. One of the most important things when watching a game is to know what part of the field is considered in bounds and when the ball is in play.
  
  

What is Out of Bounds?

• An NFL football field is 100 yards long, and the end zones account for an additional 10 yards each. The field is 160 feet wide. The entire playing surface is considered in bounds. The sidelines and endlines are out of bounds. According to the NFL Rule Book, the field is also “rimmed by a white border, six feet wide, along the sidelines. All of this is out of bounds.” Everything within the border is considered in bounds.
  
  

Players and Time

• Each team is allowed only 11 players on the field when the ball is snapped and the clock is running. The clock begins running after each possession change and subsequent snap. The clock also begins when the ball is kicked off. No additional players are allowed on the field once the clock starts running. The clock stops each time a player runs out of bounds, at the end of incomplete passing plays or when a penalty is called.
  
  

Player Substitutions

• There are no limits to the number of substitutions that can be made during an NFL game. Players, though, must leave the field on their own side between the end lines when a substitution is made and get off the field before a snap or free kick, according to the NFL Rule Book. Failure to get off the field before play resumes can result in an unsportsmanlike conduct penalty.
  
  

Timeouts

• Each team is allowed three timeouts per half, which stop the game clock. When a team calls for a timeout, the field judge stops play for 1 minute and 50 seconds for timeouts also honored by television stations; play is stopped for 40 seconds for non-TV timeouts. Players and coaches can stand on the field or along the sidelines during a timeout.
  
  

Injuries

• When an injured player is on the field the same rules apply as for timeouts. Once the clock has stopped, more than 11 players, trainers and coaches can be on the field to attend to the injured player. The injured player should be removed from the field as soon as possible, according to the rule book, and all players and other team personnel should return to the sideline before the clock is restarted.