Adjustable dumbbells consist of a metal bar whose center 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. Nowadays, many commercially sold dumbbells are available with sophisticated, and easy-to-use methods for weight increments adjustments.
Fixed-weight dumbbells are weights created in a dumbbell shape. Inexpensive varieties consist of cast iron, sometimes coated with rubber or neoprene for comfort, and even cheaper versions consist of a rigid plastic shell that is filled with concrete.