Some defensive formations use two ends and two tackles, but others use two ends and one tackle, in which case the tackle usually is called a nose tackle or nose guard. When the defensive formation uses two tackles, one might be referred to as a nose tackle and the other as an under tackle. In some situations, or more frequently in leagues for younger players, a defense might use five or more defensive linemen. In such cases, along with two ends, a defensive line might include one nose guard and two defensive tackles, two guards and two tackles, or players at positions that have other names like these.
One of the primary jobs of defensive tackles and defensive ends is to prevent the offense from running the ball successfully, either by holding their ground against offensive blockers or by avoiding the blockers and tackling the offensive player who is carrying the ball, as seen in the photo above. When the offense tries to pass, the main job of the defensive linemen is to rush the passer. That is, they try to tackle the quarterback before he or she can throw the ball, which is called sacking the quarterback, or to at least get close enough to disrupt the quarterback's throwing motion. In the following photo, the defensive players — those in yellow jerseys — are rushing the quarterback, who has the ball.
The second level of the defense is formed by the linebackers. Most of the time, the two linebackers who line up the farthest to the sides are called outside linebackers. They might line up directly behind the defensive ends or behind and a little to either side of the ends, or they sometimes are on the line and outside the defensive ends. Behind the linemen and toward the middle of the field, a defense might have one linebacker, called a middle linebacker, or two linebackers, who would be called inside linebackers. Most linebackers take their position standing up or slightly crouched, as seen in the photo below, but outside linebackers occasionally line up in three-point stances when they are on the line and outside the defensive ends.
Linebackers' responsibilities depend on the defense's strategy and whether the offense is trying to run the ball or pass it. Inside or middle linebackers typically focus more on stopping the run. If the offense tries to pass, however, they might have to cover a potential receiver, defend a particular part of the field or rush the passer. Outside linebackers typically have similar responsibilities, but in some defenses — mostly those called 3-4 defenses, which use three linemen and four linebackers — the outside linebackers are much more likely to rush the passer. Middle or inside linebackers typically are more stout than outside linebackers, who usually are taller and faster.
Outside linebackers are sometimes used on a particular side of the offense, either the strong side or the weak side, which are determined by the alignment of the offensive players. In these cases, they usually are called strongside linebackers and weakside linebackers, respectively. They also might be called "Sam" and "Will" linebackers for short, with a lone middle linebacker being called the "Mike" or the two inside linebackers being the "Mike" and the "Ted." A pass-rushing outside linebacker in a 3-4 defense might be called a "Jack" linebacker using these naming conventions.
The smallest and fastest players on the defense typically are the defensive backs, who are referred to collectively as the secondary. Cornerbacks are aligned the farthest to the outside and might be close to the line of scrimmage or as much as 10 to 15 yards (about 9.1 to 13.7 m) behind it. Safeties usually line up the farthest back and toward the middle of the field.
Most defenses use two cornerbacks and two safeties. When each safety has slightly different responsibilities, one might be called a strong safety and the other a free safety. A strong safety usually is bigger, and a free safety usually is faster. When the offense is very likely to pass the ball, the defense might replace one or two linebackers or defensive linemen with additional defensive backs. A fifth defensive back on the field usually would be called the nickel back, and a sixth would be called the dime back.
The primary job of most defensive backs is to defend against the pass, although they sometimes will rush the passer, and strong safeties might have more run-stopping responsibilities. When the ball is passed, defensive players usually try to intercept it, like the player in the gray jersey in the photo above, or to deflect it away from an offensive player. The defensive player also might try to cause the receiver to drop the ball as he attempts to catch it. If the offensive player does catch the ball, as seen in the photo below, the defensive player usually tries to tackle him as quickly as possible.
Cornerbacks typically line up directly across from the offense's wide receivers and often must follow the receivers and try to prevent them from catching passes. To cover receivers well, cornerbacks typcially need extraordinary speed and agility to react to whatever moves the receivers make while running their pass patterns. Safeties might be asked to cover the offense's running backs, its speedy wide receivers or its larger and stronger tight ends. They also might help another player cover a potential receiver, which would be called double coverage.
Like linebackers, defensive backs sometimes are asked to defend particular parts of the field, called zones, when the offense tries to pass. In zone coverage, rather than following a potential receiver wherever he or she goes, which is called man-to-man coverage, the defensive back simply covers any receiver who is in his or her particular zone. Defenses often try to confuse offenses by using man-to-man coverage and zone coverage on different plays or even combining the coverages on certain plays. Some defenses, however, will use mostly one or the other.
In some cases, a defense will try to surprise the offense by moving a player out of his or her usual position. For example, a linebacker or safety might move up to the line of scrimmage in order to more quickly charge through the offensive formation — which is called a blitz — when the play begins. The player might also fake a blitz to confuse the offense. A safety also might line up near the linebackers to better defend against the run. In some defenses, a player who frequently moves around the defensive formation is called a rover or a bandit.
Along with the appropriate size, speed and other physical attributes, players at the defensive positions in football typically must have aggressive attitudes. Defensive players must aggressively pursue the football or whoever has it, so they can tackle the player who has the ball or even gain possession of the ball by grabbing it or catching it. They also often try to hit or tackle offensive players as forcefully as possible, as seen in the photo below, which shows a defensive player in a dark jersey hitting a quarterback just after the quarterback threw a pass. In addition to inflicting some measure of pain, a hard hit sometimes causes an offensive player to drop or fumble the ball, which might allow the defense to gain possession of it in some cases.
Under the rules used in some leagues, players at each of the defensive positions in football must wear certain numbers on their jerseys. This helps the players' positions to be identified more easily. Defensive linemen typically are allowed to wear numbers 50 to 79 and 90 to 99, linebackers can wear 50 to 59 and 90 to 99, and defensive backs can wear 20 to 49. Unlike some offensive players, however, a defensive player is permitted to move to any position at any time during a game, regardless of his or her jersey number, without being called for a penalty.