Sturdy Material: The black plastic casters are made of high hardness nylon, with a thick center support to ensure they are sturdy enough for daily use. Smart Design: The 360 degree swivel degrees with dual rail wheel makes the casters more flexible, reasonable brake makes it easy to operate. Quantity Size: Package includes 4 black threaded casters and matching threaded nuts and wrench. The size is as the picture shows. Easy to install: We equipped a matching installation tool for you, just drill a 1.2cm deep hole for the threaded nut M7.5, then screw them in and connect them to finish. Wide application: plastic casters won't damage the carpet or any floors, easily keep quiet to bring your family a life quiet and convenient. So it can be widely used for furniture such as office chairs, tables, suitcases, bookcases, small carts, etc.