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Man-to-man defense is a type of defensive tactic used in team sports such as American football, association football, basketball, and netball, in which each player is assigned to defend and follow the movements of a single player on offense. Often, a player guards his counterpart (e.g. center guarding center), but a player may be assigned to guard a different position. The strategy is not rigid however, and a player might switch assignment if needed, or leave his own assignment for a moment to double team an offensive player. The term is commonly used even in women's basketball, though the gender-neutral 'player-to-player' also has some usage.
The alternative to man-to-man defense is zone defense, in which the defender is assigned a specific area of the floor, and then guards whatever offensive player enters his area.
The advantage of the man-to-man defense is that it is more aggressive than the zone defense. It also allows a team's best defender to stay on a player who has to be guarded at all times. The disadvantage is that it allows the offensive team to run screens more effectively, and it leaves weaker or slower defenders more exposed. In a man-to-man defense, those defenders are generally teammates staying close to their own assigned offensive player, and thus are often not in good position to offer help should a weaker defender be eluded by the offensive player he is trying to guard.
Following a rule change in 1977, man-to-man was the only type of defense allowed in the National Basketball Association. This, in theory, created more actions (such as drives) that were more attractive to fans. During this period, an illegal defense violation was called when a defender was either guarding an area instead of a specific offensive player, or was double teaming an offensive player away from the ball. With the rule change in 2001 permitting zone defense, defenders now have more freedom. However, a defender who is standing inside the key is limited to playing zone defense (not guarding an offensive player at arm's length) for no more than three seconds. If the defender violates this rule, a technical foul is assessed against him, and the opposing team is granted one free throw and subsequent possession of the basketball.
Man-to-man defense is still the primary defensive scheme in the NBA, and some coaches use it exclusively.
When defending the ball (i.e. guarding the man with the basketball) away from the basket in basketball, players typically should use a version of the following technique: The defender stands and faces the opponent. He is positioned between the ball and the basket and may be angled in one direction or another depending on the defensive scheme of that defender's team. He has his feet positioned beyond shoulder width with most of the weight distributed to the balls of his feet. However, the defender's heels should not be off the floor as this will put him off balance. The defender's knees should be bent at roughly a ninety degree angle with the bottom of his thighs parallel to the ground. This will place the defenders buttocks in a seated position. The defenders back should be straight with just a slight tilt forward. This will place the defender's head over the center of his body and maintain proper balance. Depending on the teachings of his coach, the defender should position his hands wide as if he were stretching his wingspan or place one hand high and one hand low.
See also 
If the athlete with the ball is dribbling with his left hand; lean your left shoulder and left foot to the right. You should do the same with the opposite side if the athlete was to go right.