What is Bas Relief and what does it have to do with quilting?

Bas relief (pronounced BAH re-LEAF) is an art term that refers to three-dimensional sculpture that emerges from a two-dimensional surface. This panel of St. Michael and the Dragon from the front of a building on my street in NYC is an example of bas relief:  (I happen to think this is amazing and hope some day to have time to try it as a quilt!)

Quilting adds the third dimension of depth to the two-dimensional surface of your pieced top, so quilting forms a sort of bas relief on the surface of your quilt.

When you're thinking about your quilting design, consider it not just in terms of lines, but also in terms of contour. What parts of the design will pop up and what parts of the design will be squished down? Leave some areas more open so they are higher and use denser fills to squish down background areas. (For even more dramatic differences in dimension try using trapunto -- google it.)