The Pitti Palace in Florence, Italy, illustrates this technique of trompe de l'oeil [in French it literally means mistake the eye, and sometimes the de, meaning of, is omitted]. Built during in the 1300s, a century before the beginning of the Renaissance, this impressive example of the illusionary technique has walls that seem absolutely three-dimensional with false balconies at the tops of these walls, statues that are really part of the murals. Even false sculptures over doorways decorate the walls.
This artistic technique was economical and useful as it created dimension and structure without the cost. For instance, in Guercino's Aurora, 1621-23, the addition of towering illusionistic piers--even a ruined one at one end of the ceiling--makes a more impressive picture.
Trompe l'oeil (which means to trick the eye) attempts to create an optical illusion where objects are represented as being three dimensional. With ceilings, we often see clouds or other celestial beings depicted as if floating above us. It is also common to see trompe l'oeil used in muralistic paintings, as well as set designs for the theater. Keep an eye on the light sources and shading zones, as they work together to attempt to give the work the three dimensional elements.