A weight plate is a flat, heavy object, usually made of cast iron, that is used in combination with barbells or dumbbells to produce a bar with a desired total weight for the purpose of physical exercise. Two general categories exist: "standard" plates, which have a center hole of approximately 25 mm (one inch), and "Olympic" plates, meant to fit on the 50 mm (two inches) sleeves of Olympic barbells. Standard plates are usually paired with adjustable dumbbells and Olympic plates with full-size barbells, although standard barbells and Olympic dumbbells exist. Weight plates may incorporate holes for ease of carrying (called "grip plates") or be solid discs (especially those used for competition). Non-competition plates often have variable diameters and widths, such as on the adjustable dumbbells pictured right, with heavier plates generally being larger in diameter, thickness, or both. Weight plates are typically round, although 12-sided each.