Many types of spatial experiences can be achieved by manipulating the elements. Line implies continued direction of movement and helps to indicate spatial presence. It can be used to unify the front, middle, and background areas. The length, width, and shape of a line can take on different spatial position and movements in contrast with one another. Overlapping lines establish different spatial positions, as do diagonal lines, which move in the picture space.
Lines also clarify the spatial dimensions of solid shapes.
The spatial principle s of line are so varied that some effects are unexpected. Vivien Abrams, Changing Dynamics, 1984.