What can a line do through a work of art?

Answer 1

Look at all the things lines do in this:
http://en.wikipedia.org/wiki/Image:Paul_Gustave_Dor%C3%A9_(1832-1883)_-_Baron_von_M%C3%BCnchhausen_(1862)_-_009.jpg

And here:
http://www.fws.gov/r9extaff/drawings/sgbirds.html
lOOK CLOSELY, and you will see it is all lines.

Lines can establish position, depth, shading, height, perspective, and in the two examples above, contours.