Line
A contour is the boundary where two  shapes come together.  Contours may be used to separate different regions of color or value within a single object, or to distinguish an object from its neighboring forms and the surrounding negative space.  Contours are either open (broken or incomplete outlines) or closed.   Closed contour drawings that have little line variation are essentially flat, like the pictures in a coloring book, but they can provide added clarity and precision by eliminating distracting detail.