Shelf is a flat, horizontal surface used to store or display items. It can be made of various materials such as wood, metal, or plastic. The term shelf can also refer to a variety of other structures, including a RACK, REEF, PROJECTION, BRACKET, BERM, BENCH, SILL, and LEDGE. A RACK is a structure with multiple levels of shelves, usually made of metal. A REEF is a shallow area of the ocean floor that is exposed at low tide. A PROJECTION is an outward extension of a structure, such as a shelf. A BRACKET is a support structure that is attached to a wall or other vertical surface. A BERM is a raised bank of earth or sand. A BENCH is a long seat with a back, typically made of wood. A SILL is a flat piece of material, such as wood or stone, that is placed at the bottom of a window frame. A LEDGE is a narrow, flat surface that projects from a wall or cliff.