Speed bags are small, air-filled bags anchored at the top to a rebound platform parallel to the ground. Speed bags help a fighter learn to keep his or her hands up, improve hand-eye coordination, and learn to shift weight between feet when punching. They are also known as speedballs or speed ball bags. They are generally filled with air and fitted around a tight PU-based or leather material. They come in various sizes, ranging from the large 13x10" (33x25 cm) and 12x9", midsize 11x8", 10x7" (25x18 cm) and 9x6", to the small 8x5", 7x4" and 6x4" (15x10 cm). Generally the larger the bag, the slower it is and the more force is required to keep it going. Large bags are used more for building strength and endurance, while smaller bags allow the training athlete to focus on faster hand speed, timing and coordination. Beginners might view this bag more as a "control bag", not a speed bag, for they will not be able to punch both quickly and repetitively until they gain control over their swinging force and speed.