The most fundamental form which can be expressed is the line. The best definition that I’ve heard for how to describe the line is from the great painter Paul Klee who wrote, “a line is a point out for a walk.” A point is a term used in art to refer to a specific location in space which has no length, width, or breadth, and is useful only in concept for indicating a place within a piece. The natural progression of the point is the line which would form if you were to trace a point as it moved. Now it is important to say here that the line is purely abstract, there is no such thing as line in nature, there is nothing that you can find which has no width or breadth, but only width. The abstraction of the line, however, is convenient for the artist to be able to talk about the rest of a piece. Now enough philosophy, on to the practical.

Types of Lines

Lines come in 3 major flavors:

Actual Lines are continuous and exist visually. An example would be a guitar string
Implied Lines are lines that are non-continuous and must be formed by the eye connecting the pieces of the line together. A queue of people waiting for tickets is an implied line as the line is not physically there, but you are able to figure out where that line begins and ends.
Psychic Lines do not exist at all, but they are formed when the mind discerns a line between objects. In the case of an arrow pointing to a door, there is no direct line between the arrow and the door, but a viewer would be able to understand that there is, in fact, a line. Psychic lines are often used in art and design to guide the eye around the piece using things like pointing fingers or eye contact (more on this later).

When approaching the transition of a 3 dimensional form like a tree onto a design or painting, the idea of line becomes extremely important. The line in a composition is rarely used on its own, but rather indicates a meeting of two values, or two surfaces.

A meeting of value would indicate shading. Think of tracks made in snow, the line is created by a difference in the value of the snow caused by different shadows. The snow is still the same color, the light is just reflecting differently.
Meetings of surface are the most common lines, as we see them all the time. When expressing an object that is one material which is surrounded by any other material (think about the outline of a person against the sky) the difference in material forms a line.
Intersects of surface define the line made when two surfaces intersect, such as a corner of a room.

So why does it all mater?

A line is a very powerful tool in any artist or designer’s arsenal. The goal of any piece is ultimately unity of the piece. What I mean is that the positive space (the space which has something in it) is equally “active” as the negative (or unworked) space. The line is able in many ways to achieve unity in a piece through a number of means, which are all incredibly important.

Defining form or space is achieved by showing the outline or contour of the form. If I were to draw a cube on a piece of paper, I would not need to create that cube by shading each side, but rather I can indicate the form by just showing the outline.
Creating direction using line is often overlooked, but is really the most fundamental function of line. To move a viewer through the piece the artist may use many lines to show where the eye should go (often through the use of psychic or implied lines).
Rhythm and sequence are both often indicated by line or line segments. Rhythm is created by the use of lines to indicate areas of emphasized activity alternating with areas of lower activity, often in a repetitious manner to form pattern. Sequence refers in this case to forms occurring at regular (or slightly irregular) intervals.

Qualities of Line

To talk about line, you need to understand the language used to describe the line. A measurable component of a line (or any element of design) is a quality.

Direction refers to where the line is going and how it is getting there. Commonly direction is talked about as vertical, horizontal, diagonal, curved, or freeform.
Weight is the thickness of the line, but more importantly it is literally weight. A thick line will appear in a piece as heavier than a thin line
Value is the lightness or darkness of a line.
Texture can be tricky to express in line (the diagram is a little extreme). The texture of a line more accurately refers to a very tactile quality of a line, does the line have extreme angles, or is it smooth?

Line in Composition

Horizontal lines have a calming effect on the piece indicating calm, stasis, and quiet. Most often the horizontal line is found in nature in places that are static or serene (like the horizon or calm water). The horizontal is also associated with the human sleep posture.
Vertical lines have a much more active effect within a piece. The vertical line most often is used to express rigidity or strength (like an architectural column, or a flagpole). The human analogy would, of course, be standing up.
Diagonal lines are the the most active and are used to indicate movement or activity. A human assumes a diagonal position when running.
Curved lines are expressive of sensuality or flowing movement. Curved lines are the most common in nature and can be seen in everything from the flow of water to the arms of galaxies. The curved line is predominantly what the human form is made of.

Each of these line types contributes to the creation of an effective composition. Using horizontal lines within the compositional space will help ground the piece, allowing the eye to break from that area. The focus of a piece is most often on the diagonal to have the highest activity directly on the main subject. The power of the diagonal line, even when psychic should not be underestimated when considering activity in the piece. Vertical lines assist in framing pieces and creating areas of solid strength. This is (of course) just a very small primer on the use of the line in the compositional space. Do not worry if you don’t get it right now, it will become clearer with time and practice. Also stay tuned for line analysis where I’ll be going through some paintings to demonstrate the concepts shown here.

I hope that you’ve learned something from all this, so good luck, and go create!