Surface orientation terminology: In orthographic projection, is a “normal surface” defined as perpendicular to a plane of projection?

Introduction / Context:
Classifying surfaces helps predict how they appear in different views. Common categories are normal, inclined, and oblique surfaces. Each has a specific relationship to the principal planes of projection and therefore a characteristic appearance in views (true size/shape, foreshortened, or edge).

Given Data / Assumptions:

• Principal planes are the frontal, horizontal (top), and profile (side) planes.
• A “normal surface” has a precise textbook meaning in drafting.
• We are discussing principal views, not auxiliary projections.

Concept / Approach:
A normal surface is parallel to one principal plane of projection and perpendicular to the other two. Because it is parallel to one plane, it appears in true size and shape in that plane’s view; in the other two views, its edges appear as lines. Therefore, the defining property is parallelism to one plane, not perpendicularity as the sole defining trait.

Step-by-Step Solution:

Verification / Alternative check:
Take a rectangular plate parallel to the front plane: its front view shows true size; top and side views collapse to edges. This matches the behavior of a normal surface and hinges on parallelism, not merely perpendicularity.

Why Other Options Are Wrong:

Common Pitfalls:
Equating “normal” with “perpendicular” by everyday language instead of the drafting definition; overlooking the requirement of one-plane parallelism.

Final Answer:
Incorrect