A continuous mark made on a surface by a moving point; it may be flat (pencil line) or three-dimensional (a rod, groove, ridge, etc.) Line may be explicit - a line painted along the edge of the road - or implied by the edge of a shape or form. Lines are used to outline (diagrammatic or contour lines), create shading and show form (structural lines, hatching and cross-hatching), decorate, express emotion, and direct the viewer's eye. Lines can be categorized as horizontal, vertical, diagonal, curved, and zigzag.
Lines can be hard, sharp, straight, geometric; they can be organic, smooth, soft, flowing, loopy, wavy.
Lines can remain a constant thickness (descriptive, analytical, objective, showing little of the action used to create them) or vary in thickness along their length (flowing, calligraphic, lyrical, showing emphasis and something of the gesture used to make them).