Thick outlines serve to separate subjects/shapes from one another and can be used throughout the piece to give a sense of division and isolation. Thick outlines around a single subject/shape might be used to give it prominence, boldness.

Thick, black outlines can allude to stained glass, so they might lend a reverential, spiritual feeling to a piece.

The outline is usually of a color and/or value that is distinct from the subjects/shapes. The color(s) chosen for the outlines might have a symbolic meaning, and varied colors can be used around different subjects to allude to a distinct aura that each subject has.

Featured: The Hour Before One Night by Paul Klee.