Which element of design is essential in drawing contour lines?

Contour lines are continuous marks that define the edges and surface ridges of a form. The element of design essential to drawing contour lines is line. A contour drawing is built entirely from lines that trace around the outline of an object and follow the curves and edges of its surface, capturing its shape without relying on shading or colour fills.

Shape is a two-dimensional enclosed area defined by lines, colour, or texture - it is the result of contour drawing rather than the tool used to create it. Form refers to the three-dimensional quality of an object (its volume and mass), which contour lines may suggest but which is itself a property of the subject, not the drawing element used. Space refers to the area around, between, or within objects in a composition. While contour drawing implies space, the actual marks on the surface are lines.

In contour drawing, the quality, weight, and direction of the line convey the character of the subject's edges, making line the single most essential design element for this technique.