Line 1-2 lies behind the frontal plane the same distance in the
horizontal and profile views, dimensions D1.Note that it is not
possible to tell if a line is a frontal line given the front view
alone. Only in the profile and horizontal views can it be
established that a line is a frontal line
Figure 7.8 Orthographic projection of frontal linesD. Horizontal Lines
A line that is parallel to the horizontal projection plane is a
horizontal line. Horizontal lines appear true length in the
horizontal view and are therefore principal lines. A horizontal
line will always be level in the frontal and profile views. If a
horizontal line is perpendicular to the frontal plane (and