Edge orientation rule: If an edge (a straight line) is perpendicular to a plane of projection, how does it appear in that view?

Introduction / Context:
Orthographic projection rules describe how lines and planes transform between 3D objects and 2D views. Recognizing when a line collapses to a point is key to interpreting drawings and constructing auxiliary views.

Given Data / Assumptions:

• An “edge” means a straight line segment on the object.
• The line is perpendicular (normal) to a particular projection plane (front, top, or side).
• Standard orthographic (parallel) projection is used.

Concept / Approach:
A line projects to its “end view” when viewed along its direction. If a line is perpendicular to a plane of projection, the projectors are parallel to the line, and the entire length collapses to a single point in that view. Conversely, a line parallel to the projection plane shows true length there; an inclined line appears foreshortened.

Step-by-Step Solution:

Verification / Alternative check:
Draw a stick normal to a wall. Looking straight at the wall, the stick’s end appears as a dot, matching the geometric rule. Rotate the stick parallel to the wall; it appears at true length.

Why Other Options Are Wrong:

• True-length line occurs only when the line is parallel to the plane.
• Ellipses are projections of circles at angles, not straight lines.
• Foreshortened line indicates inclination, not perpendicularity.
• “Invisible” is unrelated; visibility depends on occlusion, not orientation alone.

Common Pitfalls:
Confusing perpendicular vs. inclined; assuming dashed (hidden) lines based on orientation instead of actual obstruction; mixing rules for lines with those for planes.

Final Answer:
Correct: it appears as a point (end view of the line)