Line

David Hockney: CONTOUR LINE as a sensitive recorder of form and surface as it moves through space. 

Juan Gris: Very effective use of LINE VARIATION. A contour line, as seen here, can help provide information about the substance and 3-dimensionality of a form. That's why it's different from a simple outline.

Expressive line. Rhythmic line. The viewer has a sense of the speed at which the image was made.

EXPRESSIVE, OBSESSIVE line. The viewer has a sense of the speed at which the image was made, and the state of mind of the artist at the time of drawing.

IMPLIED LINE: invisible paths leading from, for example, the gaze of the madonna to the Christ child.  See textbook, pp. 82 and 83, for more on this topic.

Though it has few actual lines, this image could be described as LINEAR. That is because the edges of forms are very clear, and we follow those edges as we would follow a line (such as the edges of the hair, the drapery, and the form of Venus, the figure in the center).