A crown is a cap that is placed over a tooth and held in place by dental adhesive or cement. They are used for several reasons, including as a protective cover for badly decay or fractured teeth, permanent restoration for teeth with large fillings, and to correct minor problems in natural teeth, such as spacing or irregular shape. They can be made from plastic, ceramic, metal alloys or even a combination of metal and ceramic to maximise strength and simulate the appearance of natural teeth.