Spacers are unthreaded sleeves that fit over bolts or screws. Usually cylindrical in shape, they are employed to support mechanical, electromechanical, pneumatic and hydraulic components and to change the distance between them. Additionally, they enable adjustments in positioning relative to a fixed mounting point or frame. When metal and plastic spacers are used to provide stand-offs in this way, they may also be termed a bush. Often seen on hybrid electro-mechanical installations (such as hydraulic motion platforms) and robotic control systems, spacers are affixed by an elongated mounting bolt or screw. This passes through the spacer body and affixes it to the assembly mounting or fastening. It is, therefore, important to check the internal diameter of the spacer as well as its external measurements. Characteristically, zinc plated steel spacers have a low carbon content and moderate resistance to corrosion, making them suitable for general engineering usage. Alternatively, chrome plated steel spacers have a bright, reflective finish. Apart from steel, durable white nylon is used to manufacture plastic spacers; these offer excellent corrosion resistance.