NFL Official rules and notable rule distinctions

Although rules for NFL, college, and high school American football games are generally consistent, there are several differences. In addition, the NFL frequently makes rule changes because of exploits on the field by a single coach, owner, player, or referee.
Some of the major rules differences include:

• A pass is ruled complete if both of the receiver's feet is inbounds at the time of the catch. In the college and high school football, only one foot is required to be inbounds.
• In the NFL, a player is considered down he is tackled or forced down by a member of the opposing team (also known as "down by contact"). In college football, a player is automatically ruled down when any part of his body other than the feet or hands touches the ground or when the ball carrier is tackled or otherwise falls and loses possession of the ball as he contacts the ground with any part of his body.
• Unlike college and high school, the NFL has a two-minute warning, an automatic time-out that occurs when two minutes of game time remain on the game clock in each half of a game.
• Also unique to the NFL, the game clock never stops after the offense completes a first down in order to reset the first down chains.
• Two-point conversion tries are attempted from the two-yard line, whereas in college football they are attempted from the three-yard line.
• In college football, the defensive team may score two points on a point-after touchdown attempt by returning a blocked kick, fumble, or interception into the opposition's end zone. The NFL does not allow this, and instead a conversion attempt is automatically ruled as "no good" when the defending team gains possession of the football.
• There are several differences in enforcing penalties. For example, defensive pass interference results in the ball being placed at the spot of the foul. In college football, the same penalty is capped at maximum of 15 yards.
• In the NFL, overtime is decided by a 15-minute sudden-death quarter during regular season games and can still end in a tie if neither team scores. During college football's overtime, each team is given one possession from its opponent's twenty-five yard line with no game clock. The team leading after both possessions is declared the winner. If the two college teams remain tied, additional overtime periods are played.
• For instant replay, NFL teams are given two replay challenges per game, and can be awarded a third one if the other two are successful. Replays of scoring plays, the final 2:00 of each half and all overtime periods are instead initiated by the official in the replay booth. In college football, teams are only originally allocated one replay challenge (and can get a second one if successful), and the replay official can initiate reviews of all plays.