Most of the information that one can get when looking at a painting lies at the edge of the form. For instance when painting a tree or group of trees, most of what is within those borders is simply mass. What really describes the tree is the shape around the edges and by that outer shape it can easily be realized what type of tree it is. This is true of any subject.
I watched some very skilled artists at Disney World cut side view silhouettes of tourists using black paper and scissors. They worked because the edge described all that was necessary to convey what was within.