In sculpture, any work that projects from the background. Reliefs are classified by degree of projection. Relief sculpture is distinguished from sculpture in the round. In a bas relief the figures project only slightly and no part is entirely detached from the background (as in medals, coins, or areas of large reliefs in which the chief effect is produced by the play of light and shadow). In a haut relief sculpture, the figures project at least half of their natural circumference from the background. Between these two is the demi relief (half-relief or mezzo-relievo). The lowest degree of relief in which the projection barely exceeds the thickness of a sheet of paper is called a crushed relief. There is also a relief in reverse, called hollow relief, in which all the carving lies within a hollowed-out area below the surface plane, and which, through an illusion of depth and roundness, looks like raised relief. Reliefs may be carved from hard materials or modelled in wet clay, softened wax, or plaster. Reliefs are often elements of architectural sculpture.