American football is a unique sport -- it is a game about gaining territory as much as it is about scoring points. When two teams step onto a football field, each is battling for every inch it can take from the other. Each team wants to defend the field that is behind it and invade the field in front of it. Ultimately, they want to gain enough ground to score a touchdown or field goal.
In this article, you will learn about the field, the positions, and how points are scored, with a focus on professional football rules. We will also look at some basic techniques for the offense and defense. To learn more about football, read How NFL Equipment Works and How the Physics of Football Works.
Football is a game of inches played on a field measured in yards -- English measurements are used to track movements on the field. Teams succeed based on how many yards they accumulate or allow. An official NFL football field is a rectangle that is 120 yards (110 m) long and 53 yards, 1 foot (49 m) wide. Most fields are covered in grass and set in an outdoor (open-air) stadium. Some fields are made of artificial turf, which you'll find in many of the indoor stadiums.
The Football Field and Football
Let's take a look at some of the standard features of a professional football field:
- Sideline - The sideline is the 6-foot-wide (1.8-meter-wide) boundary line that runs the length of each side of the football field.
- End line - The end line is a 6-foot-wide boundary line that connects the two parallel sidelines. The end line and sideline compose the rectangular shape of the field. Two pylons flank the end of the end line.
- End zone - The end zones are two 10-yard-wide (9-meter-wide) areas at each end of the field inside the end line. The end zone behind a team is that team's end zone, and the end zone ahead of a team is its opponent's end zone.
- Goal line - The goal line is an 8-inch-wide (20-cm-wide) line that runs across the front of the end zone. Two pylons flank the end of the goal line.
- Yard lines and hash marks - In the 100 yards (91 meters) between goal lines, hash marks on either side of the field mark each yard. At every fifth yard, a solid white line runs from sideline to sideline, and at every 10 yards those lines are numbered (i.e., 10, 20, 30, 40, etc.). This crosshatch of lines gives the field its "gridiron" nickname.
- Goalposts - Centered at the back of the end zone is a pole that extends 10 feet (3 meters) high and connects with a horizontal cross bar. On each end of the 18-foot, 6-inch (5.5-meter) cross bar is an upright post that rises to a height of 30 feet (9 meters) above the ground. A 4-inch by 42-inch (10-cm x 107-cm) ribbon is tied to the top of each upright.
The most essential piece of equipment in a football game is the ball. Official NFL footballs are handmade by Wilson Sporting Goods Co. The football is an oblong sphere and is 11 to 11.5 inches (27.9 - 29.2 cm) long. It has a lengthwise circumference of about 28.5 inches (72.4 cm) and a width-wise circumference of about 21.5 inches (54.6 cm) in the middle of the ball. It weighs between 14 and 15 ounces (397 - 425 grams). The ball consists of an inflated, polyurethane bladder placed in cowhide covering and laced with gridcord material. Gridcord is cotton thread covered with vinyl. A three-ply, synthetic lining is sewn inside the leather covering to protect the bladder and help the football keep its distinct, elongated shape. A valve connected to the bladder protrudes through the leather and allows air to be pumped into the ball.
With the equipment identified and the field set, we can now play a game. An NFL game is divided into four quarters with an extended halftime break between quarters two and three. Each quarter is 15 minutes long. If the teams are tied after four quarters of play, they play an additional overtime period of 15 minutes. In the overtime, the first team to score wins.
While the game time adds up to one hour, it usually takes three to four hours to play a game. Teams can stop the clock by running out of bounds, throwing an incomplete pass, or calling a time-out, of which they have three per half. Time also stops for each of the two-minute warnings, observed two minutes prior to the end of the second and fourth quarters.
In the next section, we'll look at all of the players on the field.
The Football Offense
An NFL roster allows for no more than 53 players on a team. At any one time, only 11 players per team are allowed on the field. To understand an NFL roster, you have to identify the three teams within a team: the offense, the defense and special teams. Each of these groups has specialized positions with a specific set of skills. Let's take a closer look at each unit.
A team's offense is responsible for taking the ball down the field toward its opponent's end zone. To do this, the offense throws the ball from one player to another or holds the ball and runs forward. Here are the basic offensive positions.
- Quarterback (QB) - This player throws the ball to receivers or hands it off to running backs. The quarterback is also known as the "field general," because he's the on-the-field leader.
Photo courtesy The Philadelphia Eagles
- Offensive linemen - These players provide blocking for the quarterback and running backs. Individual lineman positions include Center (C), Guards (LG/RG) and Tackles (LT/RT). The Center is located in the middle of the line. This player hikes the ball to the quarterback by bringing the ball up between his legs. The Guards flank the center. The tackles are positioned on the outside of each guard. Teams have two guards and two tackles.
- Receivers - Receivers run down the field and catch balls thrown by the quarterback. Receivers are either wide receivers (WR) or tight ends (LTE/RTE), depending on where they are positioned on the field.
- Running backs - Running backs take the ball from the quarterback and run up the field. Depending on the formation (arrangement) of the offensive players, a running back might be called a tailback (TB), halfback (HB) or fullback (FB).
Photo courtesy Kansas City Chiefs team photographer Hank Young
Football Defense and Special Teams
When a team does not have possession of the ball, it is on defense and uses various methods to prevent the other team's offense from scoring. These players must tackle the offensive player who has the ball to stop the offense from advancing. Defense will also try to take the ball away from the offense.
Here are the basic defensive positions:
- Defensive linemen - The linemen put pressure on the quarterback by trying to tackle him before he releases the ball. They also try to stop running backs. There are typically three or four defensive linemen. Individual positions include Ends (LE/RE), Nose tackle (NT) and Tackle (LDT/RDT). The ends line up on the outside of the line and try to rush around the offensive tackles. The nose tackle lines up over the football. The tackle lines up across from a guard and tries to knife through the offensive line.
- Linebackers - When there are four linemen, there is a middle linebacker (MLB) and two outside linebackers (OLB). When there are three linemen, there are two inside linebackers (ILB) and two outside linebackers. Their job is to back up the linemen, as well as contain runners and cover receivers on some plays.
Photo courtesy The Philadelphia Eagles
#51 in this photo is a linebacker
- Cornerbacks (CB) - The cornerbacks prevent the wide receivers from catching the ball by breaking up passes from the quarterback.
- Safeties - The safeties play deep behind the rest of the defense to prevent a long pass or run. A strong safety (SS) lines up on the side of the field where there are more offensive players. The free safety (FS) plays a deep, middle position.
Photo courtesy The Philadelphia Eagles
If a team has to kick the ball, it uses its special-teams unit. This unit includes the team's kickers, the offensive line, and players who run down the field to tackle a returner (see below).
- Placekicker - The placekicker kicks the ball through the goalposts to score points and kicks the ball to the other team to start the game and after each scoring possession.
- Punter - The punter free-kicks the ball if his team cannot advance the ball down the field.
- Returner - During a kickoff or punt, the returner tries to catch the ball and return it as far as he can. A player can score a touchdown on a return.
In the next section, you will learn more about how teams move the ball and how the down-and-distance system works.
Moving the Football and Finding the End Zone
A football game begins with a coin toss to decide which team will receive the opening kickoff. From the opening kickoff, the two teams battle to take possession of the ball. Possession means that a team's offensive unit has the ball.
A team can take possession of the ball in several ways:
- Receiving a kickoff - A team receives a kickoff at the beginning of each half and after the other team scores.
- Turnover - A team recovers a ball dropped by the other team (fumble) or picks off a ball thrown by the other team's quarterback (interception).
- Safety - A player is tackled in his own end zone, meaning the end zone his team is defending, so the other team gets the ball though a free kick.
- Punt - The defensive team stops the offensive team from getting 10 yards in three downs, and the offensive team free-kicks, or punts, the ball to the other team on third down.
- Turnover on downs - The offensive team fails to advance the ball 10 yards in four downs and has to surrender the ball to the other team.
For those new to the sport, the last two scenarios on this list may not make sense. One of the most confusing concepts of American-style football is the down-and-distance system. Every time a team takes possession of the ball, it is given a set of four downs, or attempts, to move the ball 10 yards. If the team can move the ball 10 yards or more within four downs, the team gets another set of four downs to go another 10 yards, and so on. For instance, if a team advances 3 yards on first down, the next play is second down with 7 yards to go (second and 7); if the team then advances 5 yards on second down, the next play is third and 2; if the team then advances 2 or more yards on third down, the next play is back to first and 10, with a whole new set of four downs during which to advance the ball.
After each play, the officials determine how many yards a team has advanced or lost (a team can lose yards if the ball holder is tackled behind the line of scrimmage -- this line is discussed in a moment). The officials then place the ball at the point where the team has ended up. This point determines the line of scrimmage, which is an imaginary line that runs across the field and is the starting point for the offensive team on each play. On the sideline, a team of officials handles a 10-yard-long chain, which designates that 10-yard mark a team must reach to get a first down. On close plays, this chain is sometimes brought onto the field to measure the distance from the ball to the 10-yard mark. The nose of the ball must reach the bar connected to the end of the chain for a team to be awarded a first down.
If a team fails to gain 10 yards after three downs, it may choose to punt the ball to the other team. If it doesn't punt and chooses to use its fourth down, or "go for it," it must reach the 10-yard mark or it surrenders the ball. A team often chooses to punt the ball in order to back the opposing team up so that it has to cover a greater distance to score. The team receiving the punt can return it, meaning it can catch and run it back down the field. The kicking team is hoping to kick the ball down the field and tackle the receiving team's kick returner before he comes back down the field.
All of this pushing and shoving to move a cowhide-covered ball has one purpose: move the ball over the opponent's goal line to score a touchdown.
Photo courtesy The Philadelphia Eagles
Running the ball
The opponent's goal line is the one a team is advancing toward. Once any part of the ball reaches the edge of the goal line, it is considered in the end zone, and a touchdown has been scored. You will often hear commentators say that a ball breaks the plane of the end zone, which means the ball has crossed over the goal line.
A touchdown is just one way of scoring points in football. After scoring a touchdown, a team can kick a field goal for an extra point or attempt to run or pass the ball into the end zone for a two-point conversion. The team has only one chance at the two-point conversion.
Here is a complete look at ways points are scored and how many points are awarded for each:
|Touchdown (TD)||A ball is carried into an opponent's end zone or caught in the end zone.|
|Extra point||A ball is kicked through the uprights of the opponent's goalpost after a touchdown.|
|2-point conversion||A ball is carried into an opponent's end zone or caught in the end zone.|
|Field goal||A ball is kicked through the uprights of the opponent's goalpost.|
|Safety||A player tackles an opposing player in the opposing player's own end zone.|
After scoring a field goal or touchdown and completing the extra point or two-point conversion attempt, a team must kick the ball to the opposing team. The only exception is on a safety. A team that scores a safety gets the ball on a free kick.
Photo courtesy Pittsburgh Steelers/
In this photo you can see an official wearing black and white, just behind the action.
A football game actually consists of three teams, with the third team being the officiating crew. They also have a uniform, which consists of a shirt with vertical black and white stripes, white pants and a white or black hat. These men are responsible for enforcing the rules of the game as outlined by the NFL rules committee. An NFL officiating crew consists of six men, and each has distinct responsibilities:
- Referee - This is the head official on the field. He is responsible for giving signals and serves as the final authority on rule interpretation. If you watch an NFL game, this will be the official making announcements.
- Umpire - The umpire rules on players' equipment and conduct. The umpire takes a position about five yards behind the line of scrimmage.
- Head Linesman - The head linesman is responsible for calling infractions of player movement when lined up on the line of scrimmage. He also keeps track of the downs and manages the chain crew.
- Line Judge - The line judge keeps time during the game to backup the official clock operator. Also, he backs up the head linesman on line-of-scrimmage calls. He straddles the line of scrimmage on the opposite side from the Head Linesman.
- Field Judge - The field judge makes calls regarding the wide receivers and backs on his side of the field. He also watches the defensive players that the back is blocking. He makes calls determining if a player is in or out of bounds. He stands 20 yards away from the line of scrimmage at the beginning of a play, on the same side of the field as the Line Judge.
- Side Judge - The side judge makes calls regarding the wide receivers and backs on his side of the field. He also watches the defensive players the back is blocking. He makes calls determining if a player is in or out of bounds. He stands 20 yards away from the line of scrimmage at the beginning of the play, on the same side of the field as the Head Linesman.
- Back Judge - The back judge makes calls regarding the tight end and the player the tight end might be blocking. He is also responsible for keeping the time for the 25-second play clock, time-outs, and intermissions. He stands 25 yards beyond the line of scrimmage.
Officials must memorize and be ready to call an infraction in a split second. An official signals an infraction by throwing a yellow flag. There are many rules in the NFL Rule Book; here are a few of the ones of which you might be unaware:
- Clipping - This is a block thrown in the back of the opposing player.
- Chop block
- This is an illegal block thrown below the waist of an opposing
player. These types of blocks have been known to cause severe leg
injuries to the opposing player. The offensive team is penalized 15
yards for this infraction.
- Encroachment - A
defending player moves into the neutral zone and makes contact with an
offensive player before the ball is put in play. The neutral zone is a
space the length of the ball that separates the offense and defense
prior to a play. The only player who can legally enter the neutral zone
is the center, who hands, or snaps, the ball to the quarterback to
start a play. The offensive team is awarded 5 yards for this penalty.
- Excessive crowd noise - The referee determines that the crowd is too loud. The home team can be penalized 5 yards or can lose a time-out.
- Fair catch
- A player receiving a kick or punt can signal that he does not intend
to return the ball by putting his arm in the air. Once he signals for a
fair catch, he cannot be tackled and cannot move beyond the spot where
he catches the ball.
- Intentional grounding - A
quarterback, who is in the pocket, intentionally throws the ball away
to avoid being tackled behind the line of scrimmage for a loss of
yards. The pocket is the rounded shape formed by the offensive linemen
during a play when they are blocking for the quarterback.
- Leaping rule - While players can
block kicks, they cannot run from more than 1 yard behind the line of
scrimmage to do so. According to NFL rules, a defensive player can
run forward and leap in attempt to block a kick if he was lined up
within 1 yard of the line of scrimmage when the ball was snapped. But
if the player is lined up more than 1 yard from the line of scrimmage,
he cannot run up to the line, leap to block a kick and land on other
players. A 15-yard penalty is assessed for this infraction.
- Tuck rule
- A player, typically the quarterback, drops the ball when his arm is
moving forward to tuck the ball away. The action is considered an
incomplete pass rather than a fumble because his arm is moving forward.
- "Emmitt Smith" helmet rule - A player cannot remove his helmet on the field unless it is to adjust his equipment. This rule is dubbed the "Emmitt Smith rule" because Smith, who holds the record for most rushing touchdowns, was famous for ripping off his helmet to celebrate a touchdown. This rule was enacted to quell excessive celebrations. The team of the offending player is assessed a 15-yard penalty.
Football Instant Replay
In a modern NFL game, there are as many as 20 cameras covering the fast-paced action of a game. In 1999, the NFL added an instant replay system to back up the officials. Each game camera catches a different view of each play, and those views can be used to review questionable calls. However, not every play is reviewable.
During certain plays, coaches can challenge an official's call. The coach challenges the play by tossing a red flag on the field. Each team is allotted two challenges per game. If the team loses the challenge, it loses a time-out and the official's call stands, according to NFL rules. If a team wins the challenge, it retains its time-out and the official's call is overruled. A challenge must be made before the next play begins but cannot occur in the last two minutes of each half.
An official replay assistant can also initiate a review in the last two minutes of each half and in the overtime period. The replay assistant is not limited as to how many replays he can request.
When a play is challenged, the referee has 90 seconds to review the play. He reviews the play at a field-level monitor to the side of the field.
Here is a list of some reviewable plays:
- Scoring plays
- Pass complete, incomplete, or intercepted
- Out of bounds
- Recovery of a loose ball
Photo courtesy The Philadelphia Eagles
There's a loose ball in there somewhere.
- Illegal passes - illegal receiver or beyond line of scrimmage
- Quarterback incomplete forward pass or fumble
- Runner rule down by contact
- Touching of a kick
- Number of players on the field
To learn more about American football, as well as other sports, check out the links on the next page.
Related HowStuffWorks Articles
- Football Quiz
- How Fantasy Football Works
- How the NFL Draft Works
- How NFL Equipment Works
- How the Physics of Football Works
- How the First-Down Line Works
- How does the NFL's salary cap work?
More Great Links
- Official NFL Web site
- CBS SportsLine.com: NFL Official Signals
- Wilson Sporting Goods: The Making of a Football
- The Straight Dope: Why is a football called a pigskin?
- NFL Digest of Rules