Strategy forms a major part of American football. Both teams plan many aspects of their plays (offense) and response to plays (defense), such as what formations they take, who they put on the field, and the roles and instructions each player are given. Throughout a game, each team adapts to the other's apparent strengths and weaknesses, trying various approaches to outmaneuver or overpower their opponent in order to win the game. Offensive strategy The goal of the offense is, most generally, to score points. In order to accomplish this goal, coaches and players plan and execute plays – based on a variety of factors: The players involved, the opponent's defensive strategy, the amount of time remaining before halftime or the end of the game, and the number of points needed to win the game. Strategically, the offense can prolong their possession of the ball to prevent the opponent from scoring. Offensive scoring chances, or drives, end when they cannot move the ball 10 yards or the ball is turned over via fumble or interception. Offensive players On offense, there are three types of players: linemen, backs, and receivers. These players' positions and duties on the field vary from one offensive scheme to another. The position names (as well as the abbreviations recognized by coaches, players, and fans) vary from one team's playbook to another, but what follow are among the most commonly used: Linemen (This is understood to be players on the line other than at the ends; also referred to as "interior linemen". The ends—i.e., the players at the ends of the line—are discussed below under "Receivers".) Center: The Center ("C") is the player who snaps the ball to the quarterback. Like the other four linemen, his job consists of both run blocking (pushing the defenders away from the ball carrier on a running play) and pass blocking (stopping the defenders from disrupting or tackling the quarterback). The center is also usually responsible for calling the blocking schemes on the line, telling the other linemen which defenders to block. Guard: Guards ("G") line up on both sides of the center. The guards are generally bigger than the center and are typically better run blockers than pass blockers. Tackle: Tackles ("T") are the "bookends" of the offensive line. They are usually the biggest offensive linemen (at the NFL level they are typically at least , and can be as tall