Excerpt from Kallax Shelving Unit Cabinet :
Shelving units are made of various materials, ranging from plastic, iron, glass, to all types of wood. The design is very diverse and varied so there is no need to run out of ideas to use the shelving unit as one of the decoration choices. There are thousands of different styles you can find, from classic styles to modern ones. There is a shelving unit designed to accommodate other shelving units and some are designed to be stacked above other shelving units.