Angles appear in true size on a drawing only when the plane containing the angle is oriented how with respect to the plane of projection? Choose the correct plane relationship required for true-angle measurement.

Introduction / Context:
In descriptive geometry and orthographic projection, lengths and angles appear true only under specific orientation conditions. Understanding when an angle is shown in its true magnitude is essential for accurate measurement and clear communication on engineering drawings.

Given Data / Assumptions:

• An angle lies in a certain geometric plane.
• We wish to see the angle in true size on a given view.
• We must relate the angle’s plane to the plane of projection (e.g., front, top, or side plane).

Concept / Approach:

True size occurs when the object’s relevant feature (a line, distance, or an entire plane containing an angle) is parallel to the projection plane. If the plane is inclined or perpendicular to the plane of projection, foreshortening occurs and the angle will not be shown in true magnitude. Aligning the plane parallel ensures no perspective or projective distortion in that particular view.

Step-by-Step Solution:

Verification / Alternative check:

Engineering graphics references emphasize that distances and angles are true only when parallel to the plane of projection. Auxiliary views are often constructed specifically to achieve this condition for inclined features.

Why Other Options Are Wrong:

Perpendicular: produces maximum foreshortening, not true size.

Adjacent and aligned (as written here): are not standard projection relationships yielding true size; the necessary condition is parallelism.

Common Pitfalls:

Assuming an angle that looks “about right” is true size without checking the plane’s orientation. Always confirm parallelism or construct an auxiliary view.

Final Answer:

Parallel