A contour line follows an edge. A cross-contour line runs across a surface, following its curve, like the stripes on a beach ball, the rings on a tree trunk or the lines of latitude on a globe.
Key idea
Cross-contour lines curve with the surface. On a flat surface they run straight; on a round surface they bend.
Why it works
A straight line says "flat." A line that bends says "this surface curves away from me." When the stripes around a can bow downward, you see that you are looking down on it, because the near side of each ring sits lower on the page than the far side.
Worked example: Stripes on a can
You draw a soup can seen from a little above, and want stripes around it. Which way should each stripe bend?
- You see the can from above, so its bottom edge is a curve bowing downward.
- Stripes go around the can just like the bottom edge does.
Answer: Each stripe bows downward, matching the curve of the bottom.
Common mistake
Many people draw cross-contour lines straight across a round form, and it turns flat. Before you draw a line across a surface, trace the surface in the air with your finger and feel how it bends.