The contrast of dark and light areas gives definition to the object.
Imagine the vertical surface of a wall lit by sunlight at an angle of about 45 degrees above the wall.
Coving - theory:
At their simplest, moldings are a means of applying light- and dark-shaded stripes to a structural objects without having to change the material or apply pigments.
Adding a small overhanging horizontal molding to the surface of the wall will introduce a dark horizontal shadow below the molding, which in consequence is called a fillet molding.
Adding a vertical fillet to a horizontal surface will create a light vertical shadow.
Graded shadows are possible by using moldings in different shapes: the concave cavetto molding produces a horizontal shadow that is darker at the top and lighter at the bottom; an ovolo (convex) molding makes a shadow that is lighter at the top and darker at the bottom. Other varieties of concave molding are the scotia and congé and other convex moldings the echinus, the torus and the astragal.
Placing an ovolo directly above a cavetto forms a smooth s-shaped curve with vertical ends that is called an ogee or cyma reversa molding.
Its shadow appears as a band light at the top and bottom but dark in the interior.
Similarly, a cavetto above an ovolo forms an s with horizontal ends, called a cyma or cyma recta.
Its shadow shows two dark bands with a light interior.Źródło: https://en.wikipedia.org/wiki/Molding_(decorative)#Theory
Ceilings have frequently been decorated with fresco painting, mosaic tiles and other surface treatments. While hard to execute (at least in place) a decorated ceiling has the advantage that it is largely protected from damage by fingers and dust. In the past, however, this was more than compensated for by the damage from smoke from candles or a fireplace.
Many historic buildings have celebrated ceilings.
Perhaps the most famous is the Sistine Chapel ceiling by Michelangelo.
Ceiling height may have psychological impactsŹródło: https://en.wikipedia.org/wiki/Ceiling#Elements
Ceilings are classified according to their appearance or construction. A cathedral ceiling is any tall ceiling area similar to those in a church. A dropped ceiling is one in which the finished surface is constructed anywhere from a few inches or centimetres to several feet or a few metres below the structure above it. This may be done for aesthetic purposes, such as achieving a desirable ceiling height; or practical purposes such as acoustic damping or providing a space for HVAC or piping. An inverse of this would be a raised floor.
A concave or barrel-shaped ceiling is curved or rounded upward, usually for visual or acoustical value, while a coffered ceiling is divided into a grid of recessed square or octagonal panels, also called a "lacunar ceiling".
A cove ceiling uses a curved plaster transition between wall and ceiling; it is named for cove molding, a molding with a concave curve.1 A stretched ceiling (or stretch ceiling) uses a number of individual panels using material such as PVC fixed to a permieter railŹródło: https://en.wikipedia.org/wiki/Ceiling.