What is a Line?
Line, from the perspective of Design, is a continuous mark on a page. A line may be curved, straight, vertical, horizontal. Really, lines can go any which way. Even zig zag.
The line is a foundational element of design: the human eye, seeking patterns in a sensorium flow of imagery, locks on to lines. As humans, we trace the lines and we think in patterns.
Artists, Designers, Writers . . . use lines symbolically. People interpret lines just as they interpret alphabetical letters. Thus, lines are semiotic: they are way of communicating something to readers, listeners, users
This depiction of the Brussels Metro #5 Subway Line illustrates the semiotic nature of lines: