C. Frontal Lines
A line that is parallel to the frontal projection plane is a frontal
line. Frontal lines are principal lines and always appear true
length in the frontal view. A frontal line can be inclined, level,
or vertical, but must be true length. A vertical frontal line
appears as a point view in the horizontal view and true length
in both the frontal and profile views, therefore a vertical line is
both a frontal and profile line since it is true length in both of
these views. If a level frontal line appeared as a point in the
profile plane, it would be true length in the frontal and
horizontal views and is therefore a horizontal as well as a
frontal line.
Figure 7.7 Frontal lines