The round pen, sometimes called a bullpen, is a round enclosure used for horse training. They range in diameter from a minimum of 30 feet (9.1 m) to a maximum of 100 feet (30 m), with most designs 50 to 60 feet (15 to 18 m) in diameter. Footing is usually sand or other soft dirt. The sides are 6 to 8 feet (1.8 to 2.4 m) high, traditionally made of wooden posts with rails or wooden boards, although modern round pens are often made out of portable pipe panels that allow the pen to be made bigger or smaller, or to be moved.