Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. Within the sport of association football, a defender is an outfield player whose primary role is to prevent the opposition from scoring. There are four types of defender - centre back, sweeper, full back, and wing back. The job of the centre back, centre half, central defender, or stopper is to stop opposing players, particularly the strikers, from scoring, and to bring the ball out from their penalty area. As their name suggests, they play in a central position. The position was formerly referred to as centre half, although the emphasis of the centre half was more forward thinking in action. In the early part of the 20th century, when most teams employed the 2-3-5 formation, the two players at the back were called full backs and the row of three players in front of them were called half backs. As formations evolved, the central player in this trio, the centre half, moved into a more defensive position on the field, taking the name of the position with them. The right and left players in the trio were called the right half and left half respectively.