A barbell is a piece of exercise equipment used in weight training, weightlifting and powerlifting. It consists of a metal bar, one or more sets of weight plates, and a set of collars. Barbells range in length from 4 feet (1.2 m) to 7 feet (2.1 m). The central portion varies in diameter, but is close to one inch (2.54 cm), and is engraved with a knurled crosshatch pattern to help lifters get a good grip. Disc weights (plates) are slid onto the outer portions of the bar to obtain the desired total weight. These weights are often secured with collars to prevent them from sliding off during the exercise (which can result in injuries) or fling the unevenly-loaded barbell through the air.