punch both quickly and repetitively until they gain control over their swinging force and speed. A boxer normally hits the speed bag from the front with his or her fists, but it is also possible to use fists and elbows to hit the bag from all around it, including the front, back and sides. In this method the user may perform many diverse punching combinations that create improvised rhythmic accents. Although speed bags are normally hung vertically, recently the additional method of hanging a bag horizontally on a wall has regained popularity. This was very popular during the early twentieth century, specifically the 1920s–1940s era. The same punching skills may be used on the horizontal bag that are used when it hangs vertically. A coordination bag is a new type of speed bag that moves unpredictably rather than rhythmically. Additionally, due to the tedious mounting and anchoring necessary when installing a traditional speed bag platform, a portable speed bag platform that installs in a doorway has been created. Designed so that downward pressure stabilizes this portable speed bag platform, it can be installed and removed through a tension system that allows for use in any doorway. Swerve balls/floor-to-ceiling balls/double-end bags are almost the same as speed bags, with the only differences being that the bag' size, shape and material may be different, and that the cable system is attached to both the ceiling and a clip on the floor — when the boxer makes any strike on the ball, it reacts by swinging fast towards them, the object being to swerve, punch, dodge and improve co-ordination. The harder and faster these bags are hit, the more they rebound and react in different motions and angles, thus giving broader practice to the fighter. Double-floor to ceiling balls which allow for training body-head combinations also exist. Maize bags or slip bags are not punched with great force, but are used in boxing training to improve the athlete's head motion and ability to evade an opponent's punch, their name deriving from the fact that traditionally they are filled with maize. A heavy bag is a larger, cylindrical bag, usually suspended by chains or ropes and used for practicing powerful body punches, and can be used to toughen hands or any other limb used to hit the bag. Heavy bags are for developing power; technique is best learned on the punch mitts or pads.