Bold lines emphasize the shape or figure they represent, drawing the viewer's eye to the artwork's center of interest, possibly bringing simplicity and clarity to a design surrounded by complex detail. Lines can also create emphasis by their placement. By putting a few lines in an otherwise "dead," under-illustrated area of a drawing, an artist makes them stand out.
The juxtaposition of opposites in a work of art excites the viewer's eye and mind and line work creates this excitement. Horizontal and vertical lines create a static feeling, while diagonal lines suggest movement and depth. Carefully chosen diagonals against an otherwise static composition lend power to the image or shape they represent. Curly or wavy lines may create a pleasing contrast to, or break the visual monotony of, a composition filled with straight lines.
Unity in a work of art, if overdone, creates the impression that every design element is the same as every other element and equally important. A landscape drawing of a city or a suburban mall might comment on the drab sameness of the scene by using the same line width and style throughout the composition. If done in a minimalist style to depict something more attractive, like a field of flowers, the effect can be more pleasant.
An artist creates a feeling of harmony by bringing a complex range of visual ideas and techniques together in a pleasing composition. He might coalesce visual elements through repetition of lines--using the same cross-hatching techniques and patterns, for example, to shade the clouds in the sky, the trunks of the trees and the blades of the grass. Lines may suggest harmony by complementing or balancing each other. Here, a large curve in a line in one area of the composition plays off a slight bend in a line somewhere else; there, a thick, expressive stroke is "answered" by a demure, retreating line.