Surface intersection concept — In solid geometry and engineering drawing, is an edge the locus created where two surfaces (faces) intersect?

Introduction / Context:
Edges, faces, and vertices form the topological backbone of engineering models and drawings. Knowing what constitutes an edge ensures proper sectioning, hidden-line removal, and dimensioning of features such as chamfers and fillets.

Given Data / Assumptions:

• Statement: an edge is formed at the intersection of two surfaces.
• Surfaces may be planar or curved.
• Edges can be straight or curved depending on the surfaces involved.

Concept / Approach:
In boundary representation (B-Rep) models, faces are bounded by edges, and edges meet at vertices. When two faces intersect, the intersection curve is recorded as an edge in the model, which the drawing projects as visible or hidden lines depending on view orientation.

Step-by-Step Solution:
1) Identify two intersecting faces of a solid.2) Compute their intersection: the resulting curve/line is the edge.3) Conclude: the edge definition matches the statement.

Verification / Alternative check:
Common examples: the border of a box face meets adjacent faces along straight edges; a cylinder intersecting a plane yields a circular edge in the model where the faces meet.

Why Other Options Are Wrong:

• Incorrect: Conflicts with standard geometric/topological definitions.
• Only true in wireframe modeling: Also true in solid modeling; wireframe merely displays edges.
• Depends on projection angle: The existence of the edge is independent of view; only visibility changes.
• True only for curved surfaces: Straight edges occur between planar faces as well.

Common Pitfalls:
Confusing silhouette (outline in a view) with true model edges; misreading hidden edges due to line-weight conventions; ignoring tangency where edges may vanish (e.g., smooth fillets).

Final Answer:
Correct