The dumbbell, a type of free weight, is a piece of equipment used in weight training. It is usually used individually or in pairs, with one in each hand.Adjustable dumbbells consist of a metal bar whose centre portion is often engraved with a crosshatch pattern (knurling) to improve grip. Weight plates are slid onto the outer portions of the dumbbell and secured with clips or collars. Shown to the right is a "spinlock" dumbbell, whose ends are threaded to accept large nuts as collars. Alternatively, a dumbbell may have smooth ends with plates being secured by a sprung collar.Dumbbells are often used for joint-isolation exercises such as biceps curls, chest flyes or shoulder raises. Using dumbbells for full-body, multiplanar movements, however, can provide a variety of different strength outcomes. It also offers many benefits for cardiorespiratory fitness and flexibility.