Line is one of the formal elements of art. Along with elements like colour, shape and texture, it is something aesthetic that is an indispensable part of the design process.
Objectively, artists use line to outline shapes and create perspective, among other things. But there is also a theory that suggests various emotional states can be inspired by different types of lines in art and design. For example, a horizontal line supposedly conveys restfulness, a vertical line spirituality because it infers height, mixed horizontal and vertical lines stability, diagonal lines motion and curved lines supposedly convey humanity and sensuality. It is unknown whether people actually experience such feelings when looking at lines but both Kandinsky and Mondrian believed in the communicative power of line to express something harmonious and universal and they (still) have many followers.