Here are the key points about lines from the document:
- A line is the path of a moving point in space and is one-dimensional.
- Contour lines follow the outlines and interior details of a subject using continuous lines without picking up the drawing tool. They help show form.
- Line quality, by varying the thickness of lines, can imply shadows and add detail to suggest form.
- Cross contour lines, when added to contour lines, further communicate the three-dimensional form of a subject by indicating planes, contours, and shadows.