A down is a period in which a play transpires in gridiron football. The down is a distinguishing characteristic of the game compared to other codes of football, but is synonymous with a "tackle" in rugby league. The team in possession of the football has a limited number of downs (four in American football, three in Canadian football) to advance ten yards or more towards their opponent's goal line. If they fail to advance that far, possession of the ball is turned over to the other team. In most situations, if a team reaches their final down they will punt to their opponent, which forces their opponent to begin their drive from further down the field; if they are in range, they might instead attempt to score a field goal.
Description
A down begins with a snap or free kick (such as a kickoff or safety kick), and ends when the ball or the player in possession of it is declared down by an official, a team scores, or the ball or player in possession of it leaves the field of play.
The player with possession of the ball after he has been tackled or is otherwise unable to advance the ball further on account of the play having ended is down (e.g., "He is down at the 34-yard line").
Down may also refer to the ball after it is made dead in one manner or another. The line of scrimmage for the next play will be determined by the position of the ball when it is down.
Each possession begins with first down. The line to gain is marked 10 yards downfield from the start of this possession, and the situation is described as "1st and 10" (if the goal line is less than 10 yards downfield, then the goal line is the line to gain and the situation is "1st and goal"). If the offensive team moves the ball past the line to gain, they make a new first down. If they fail to do this after a specified number of downs (four in American play and three in Canadian play), the team turns the ball over on downs, and possession of the ball reverts to the opposing team at the spot where the ball was downed at the end of the last down.
If a penalty against the defensive team moves the ball past the line to gain, the offensive team gets a new first down. Some defensive penalties give the offense an automatic first down regardless of the distance.
When the offensive team reaches the final down, the team faces a last down situation (third down situation in Canadian play and fourth down situation in American play), where the team must decide whether to use a running or passing play in an attempt to gain a first down (this is called going for it), or alternatively to kick the ball (either by punting or attempting a field goal). Though statistical analysis of games suggests playing more aggressively is the better option, kicking the ball is typically seen as the safer solution; scrimmaging may lead to a turnover on downs, potentially giving the ball over to the other team with good field position.
Downing the player with possession of the ball is one way to end a play (other ways include the player with the ball going out of bounds, an incomplete pass, or a score). Usually a player is made down when he is tackled by the defense. In the NFL, if the offensive player is touching the ground with some part of his body other than his hands or feet, then he is down if any defensive player touches him. In the NCAA, an offensive player touching the ground in the same manner is down, regardless of whether a defensive player touches him.
If recovering the ball in one's opponent's end zone (following a kickoff in American football, and following any kick into the end zone, except for successful field goals, in Canadian football), a player may down the ball by dropping to one knee (in Canadian play, doing so scores a single for the opposing team). A player in possession of the ball will down the ball if he fumbles it out of bounds. If a quarterback is running with the ball during his initial possession following the snap, he may down the ball by doing a foot-first slide this is to protect the quarterback from injury. In the NFL, the quarterback is the only player for whom falling down in this way automatically stops play.
Terminology
The situation at a down can be described succinctly in a short phrase of the form 1st/2nd/3rd/4th and X. The first part describes which down the offense is on, and the X is a number of yards between the current line of scrimmage and the line at which the offense would gain another set of downs. Thus, offenses will normally begin on 1st and 10. If they gain 5 yards on the play, the subsequent situation would be described as 2nd and 5.