Horizontal suggests calmness, stability and tranquillity. Vertical gives a feeling of balance, formality and alertness. SIZE Size is simply the relationship of the area occupied by one shape to that of another. A large shape close to the center can be balanced by a small shape close to the edge. Gradation of of colour from warm to cool and tone from dark to light produce aerial perspective.
Gradation can add interest and movement to a shape.
A gradation from dark to light will cause the eye to move along a shape. The major contrast in a painting should be located at the center of interest. Too much contrast scattered throughout a painting can destroy unity and make a work difficult to look at. Unless a feeling of chaos and confusion are what you are seeking, it is a good idea to carefully consider where to place your areas of maximum contrast. Unity in a painting also refers to the visual linking of various elements of the work.