The separating line introduces geometric related spaces to the canvas: it gives identity to the field by separating it into different spaces. It also tells the viewer how the spaces relate to each other. The separating mark should be treated as a space itself with breadth and depth that is used to relate objects in a drawing.
Contour lines suggest that the fields contain different objects. They can be considered silhouettes or outlines. It gives no more information than this.
Shadow is how the human eye recognizes separated objects. We often suggest shadows in our drawings with marks, which lets the viewer recognize the objects. But shadow is only one type of separation line.
Close and repeated use of shadow lines can suggest texture. Each object in the scene is separated from the others by marks that represent shadows.
Hatching marks are often used to indicate shadow, typically parallel and of similar size. These marks might indicate texture, darker shadows from the material within the shadow. Or they might represent some other message about the object.
Albrecht Dürer pioneered the use of hatching as a composition device in his art. He made directional lines to help direct eye movement toward important obejects. Cross-hatching, lines of criss-crossed direction, and stippling don't move the eye at all.
It is essential for hatching lines to relate to each other, to unite the shadow as a whole line with a geometric isomorphism by being very repetitive and with little variance.
Through the use of shading and hatching, the illusion of white lines can be used in drawing. The shaded lines establish a negative space in order to portray the object.
A balance of positive and negative space can be achieved with careful use of shading to create white lines. Advanced artists will use these white lines when drawing just as much, if not more, than when painting.
Lines can be used to represent objects themselves. Representative lines have been used since early history as written languages because it is so quick and easy to create them. Early hieroglyphics were further abstracted from representative shapes to the quick written languages we use today. Our written languages don't very much represent any natural objects, but portray concepts or pieces of words.
Written language is actually a type of drawing. Also, all drawing is a type of language. Each line has a representative meaning that communicates some concept.
Egyptian hieroglyphics incorporated natural drawings using separating lines and texture alongside quick and abstract written language, representative lines.
Lines of grouped objects
The original Latin word for line meant "string, row, series."
In order to suggest projective space, objects must be composed in a logical order. All marks, whether they be separating lines or representative lines, must be arranged into lines in order to make sense. In all of the world's written languages, the marks we make in written language are arranged in lines.
Marks that suggest separation of objects in a drawing likewise are arranged in a logical linear order.
It is important to arrange objects in a line, not only to establish the illusion of linear perspective, but also make the spaces morphic. If we don't repeat objects to form logical lines, we only have what is called affine space. Affine space is like connect-the-dots, where you only have a bunch of points and possible vectors between them. But if you draw lines between the points by repeating objects, then suddenly you get the representation of something.
The Golden Mean is used to arrange objects into larger objects, the spacing of trees in a forest. It is the minimum ratio of mass to spacing by which smaller objects can make up a larger object.
The space between objects suggest time, the time it takes to travel from one object to another. Movement along this line can be suggested by the hatching. This suggests the transformation of an object into smaller objects, or smaller objects into a large formal object. Hatching lines can thus be used to suggest the creation or death of objects within a composition, as well as their relationship to other objects.