Form: the appearance of having three-dimensions (3-D), whether the artwork is two-dimensional or three-dimensional.
Three-dimensional (3-D) objects have height, width, and depth. Form deals mostly with three-dimensional art, but also applies to two-dimensional art in the respect that we can make objects look like they have form by using value (shading - a gradual change in value). This change is used to create the illusion of three-dimensional form.
To illustrate three-dimensionality, look at the images* below (click on the image to enlarge it):
The subtle variations in value give the illusion of depth. It looks like we could almost reach in and pet the horse. The line drawing (right) looks flat (two-dimensional), and we are not fooled into thinking the horse has form. However, in the second image, the horse is taking on more depth by using shading. In the final image (far right), the horse is now a 3D sculpture.
*Images
