A. Principal Lines
♦ A horizontal plane: it is parallel to the projection
plane. It is true size or shape in the horizontal view
since all of its lines are principal lines, therefore they
project true length. The frontal and profile views of a
horizontal plane always show the plane as edge view.
♦ A frontal plane: it lies parallel to the frontal projection
plane where it shows as true size. In the horizontal
and profile views the plane appears as an edge view.
All lines show true length in the frontal view, since
they are principal lines (frontal lines).
♦ A profile plane: it is true size in the profile view and
appears as an edge in the frontal and horizontal
views. Every line in the plane is true length in the
profile view since they are profile lines. Profile planes
are perpendicular to the frontal and horizontal
projection planes.Figure 7.19 Types of principal planes