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How does an outline sketch communicate the shape of an object?

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Watercolor painting by Berthe Morisot showing Edma seated on a sofa in a floral dress with her young daughter Jeanne beside her, both facing slightly right. — Drawing *The Artist's Sister, Edma, with Her Daughter, Jeanne*, watercolor over graphite on paper by Berthe Morisot, 1872; in the collection of the National Gallery of Art, Washington, D.C.

An outline sketch communicates the shape of an object by using line—an abstract element that acts as a border between bodies, colors, or planes—to represent the object's outer limits.1

The visual interpretation of these lines relies on the viewer's ability to associate specific linear forms with physical reality. For example:
* Angular meetings of two lines can represent the borders of a plane.1
* Additional lines can suggest the presence of a cubic body.1
* Vaulting lines may stand for arches, while convergent lines can suggest depth.1

By reducing complex spatial objects into a simple vocabulary of lines on a plane, an artist can distill comprehensible images. While a basic outline defines the silhouette or figure of an object, artists often elaborate on these sketches by varying line thickness or using techniques like hatching to further clarify plastic relationships, volume, and light phenomena.12