Projection plane concept: A projection plane is an imaginary two-dimensional plane used to create views. Must it always be parallel to the object’s surface to be valid?

Introduction / Context:
Understanding projection planes is essential to reading and creating orthographic views. A common misconception is that a projection plane must be parallel to a surface of interest. In reality, orientation affects appearance (true size vs. foreshortened), but the projection itself remains valid regardless of parallelism.

Given Data / Assumptions:

• We use parallel (orthographic) projection for principal views.
• Any plane can serve as a projection plane (front, top, side, or auxiliary), chosen for clarity.
• “True size” is visible only when the surface is parallel to the projection plane.

Concept / Approach:
If a plane surface is parallel to the projection plane, it appears in true size and shape. If it is inclined, it appears foreshortened. If it is perpendicular, it collapses to an edge. Therefore, a projection plane does not need to be parallel to the surface; rather, parallelism is a special case used when true shape is required. Auxiliary views are often added precisely to make a selected surface parallel to a new projection plane and recover true size.

Step-by-Step Solution:

Verification / Alternative check:
Construct an auxiliary view whose plane is parallel to a previously foreshortened surface. The new view displays the true shape, confirming that orientation governs appearance rather than the validity of projection.

Why Other Options Are Wrong:

• “Must be parallel” confuses “true size” with “valid projection.”
• “Must be perpendicular” is equally unfounded.
• “Tilted 45°” is not a universal rule; 45° miter lines are drafting aids, not projection-plane requirements.
• Coinciding with the centroid is irrelevant to projection geometry.

Common Pitfalls:
Assuming a surface is wrongfully drawn if foreshortened; forgetting to add auxiliary views for true-size needs; mixing rules for edges and planes.

Final Answer:
Correct: it need not be parallel; parallel gives true size, other orientations give foreshortening