example (3), line1-2 is parallel to both the profile and
horizontal projection planes and appears as a point view in
the frontal view, and true length in both other principal views.
Hence, line 1-2 is a profile line, and a horizontal line.
Vertical lines are perpendicular to the horizontal plane (view)
and appear true length in both the frontal and profile views
and as a point in the horizontal view. Thus, a vertical line is
parallel to the profile and frontal projection planes.
Figure 7.5 Level and Vertical line