Question

Outer curve of an arch is called -

• A. Intrados
• B. Evolute
• C. Involute
• D. Extrados

Hint:

In arch design, always distinguish between the intrados and extrados for understanding stress distribution.

Answer 1

The Correct Option is D

The Latin prefixes settle this quickly: "intra-" means inside (intrados, the inner curve) and "extra-" means outside (extrados, the outer curve), while evolute and involute are unrelated mathematical curve terms borrowed from geometry, not arch vocabulary.

Since the question asks for the outer curve specifically, the "extra-" (outside) term is the one that fits.

Therefore, the correct answer is Extrados.

Answer 2

Sorting the four terms by which discipline they actually belong to also isolates the answer.

1. Intrados: A genuine architectural/structural term, but it names the inner face of the arch, not the outer one asked about.
2. Evolute: Belongs strictly to mathematical curve theory (calculus of curvature), never used to label a physical arch surface in construction drawings.
3. Involute: Also belongs strictly to mathematics (and mechanical gear-tooth profiles), not to arch construction vocabulary.
4. Extrados: A genuine architectural/structural term describing the outer curved surface of an arch or vault.

Once the two purely mathematical terms are ruled out as not belonging to arch vocabulary at all, the choice is between the inner and outer architectural terms, and the outer one is extrados.

Therefore, the correct answer is Extrados.