Structural lines in art define the shape and directions that make up a drawn form. While those who relied heavily on structural lines, such as 20th century French artist Marcel Duchamp, are extremely well-known, adherents of the elements of art used in formal analysis often overlook the importance of these lines, because they aren't visible in completed work. Such lines, however, reveal how planes connect to create a sense of volume and give an artwork dimension.
All structural lines have a direct bearing on the sense of space in an image. Horizontal and diagonal structural lines create a shallow sense of space, while vertical lines convey deeper space. Sometimes, the artist incorporates a structural line into the subsequent phase of line use in a piece of art, known as the conjunctive line. Other times, they appear with organizational and contour lines. In these processes, the artist can integrate structural lines into the image to the extent that they no longer appear as detectable lines. Sketches and drawings, however -- like those of Duchamp -- typically retain traces of the structural lines or use them to create an abstract effect.
In figure drawing, structural lines organize the human body according to significant changes in plane, values and volume. The curved lines that run vertical and horizontal and cross near the nose in an early sketch of a human face are considered structural lines, and lend the drawing a sense of depth. These lines remain visible when the figure drawing remains a sketch, but disappear when the artist adds layers or uses the sketch as a guide for subsequent work in another medium, such as painting.
The term "structural lines" also refers to a technique most often employed on scratchboard, a paper coated with clay. The paper is "scratched" with the lines, which leave white marks and resemble a print or stamp. Artists similarly employ such marks in woodcuts. In both scratchboard and woodcuts, the structural lines often contrast with the tone or color of the material, which serves as the background.
Within the elements of art -- line, shape and form, space, color and texture -- structural lines rank as a subcategory. Art instructors and critics often associate or compare these elements with balance, emphasis and movement, which make up the principles of design. They further organize the category of line into directional categories, such as horizontal, vertical, horizontal and vertical combined, and diagonal lines; functional categories such as contour, structural, mechanical, lyrical, implied and blurred; and formal categories, such as straight, curved, thick and thin.