Three-dimensional works of art in which images and forms are produced in relief, in intaglio, or in the round. The term refers particularly to art works created by carving or engraving a hard material, by molding or casting a malleable material (which usually then hardens), or by assembling parts to create a three-dimensional object. It is typically used to refer to large or medium-sized objects made of stone, wood, bronze, or another metal. Small objects are typically referred to as "carvings" or another appropriate term. "Sculpture" refers to works that represent tangible beings, objects, or groups of objects, or are abstract works that have defined edges and boundaries and can be measured. As three-dimensional works become more diffused in space or time, or less tangible, use appropriate specific terms, such as "mail art" or "environmental art."