2-D design applies two dimensions: length and width. Several elements and principles of art can be applied to 2-D design to make the simplest image more interesting and complex.
Length and width of design refer simply to the horizontal and vertical aspects of the piece. An artist or designer can apply artistic elements and principles that change the nature of 2-D design. The elements are lines, shapes, values/tones, texture and color. The principles are balance (symmetry versus asymmetry), repetition/rhythm (repeating an element or creating flow and pattern), focus (designating a center or dominant area to which all other elements point, usually the spot just slightly above and to the right of the actual center), unity/harmony, scale (size), proportion, contrast (pitting opposite ideas against each other), movement (how the artist moves the viewer's eye around the piece) and depth (created by overlapping images and applying changes in scale, among other tactics). Other 2-D design considerations are the plane (size and shape of canvas), closed versus open composition (images look contained within the borders versus images that seem to flow off the edges), gestalt (the relationship of the elements to each other), positive and negative space (areas of focus and areas of passive attention), and point of view (perspective).