There are two types of defender lines. The first one is a back three. If it is not applied in a man-to-man defence, the back three does not include a libero in the middle of the line. With a free defender a back three cannot be used as a chain that moves to the side where the ball is at one moment. A modern back three is usually an oscillating back four. That means that the centre-back, who is the closest to the ball, becomes the full-back situationally, while the wing-back on the other side moves back and becomes also a full-back. Only in a few cases does a team play with a real back five when defending at the own penalty box.
A customary back four, however, constantly consists of two centre-backs and two full-backs. When defending in the middle, the back four tries to stay compact horizontally to avoid through balls. When defending attacks through a wing, the back four moves to that side. The full-back on the other side only sometimes positions wider to defend long diagonal passes, with a central midfielder moving back. Since there is no man-to-man marking and no free defender, the players of a back four can switch zones and should protect each other. While being in ball possession, the distance between each defender is bigger. As the full-backs also advancing, it looks like an accordion.