A line is a dot that went for a walk.
Man, alone in the animal kingdom, stands upright, so the vertical line represents the physical symbol of the number One, man striving toward Spirit.
This simple line is the basic shape of the World Tree or Axis Mundi that connects the Heavens, the Earth and the lower regions. It is not only a basic phallic symbol but also signifies the soul that strives for union with the Divine.
The upright line tells us where we are at a precise moment; think of the big hand of the clock, vertically oriented at 12 o’clock.