Masonry arches terminology: the under surface (concave face) of an arch is specifically referred to as which of the following?

Introduction / Context:
Precise vocabulary in arch construction avoids confusion during setting-out and inspection. Several terms describe different parts of an arch ring and its geometry.

Given Data / Assumptions:

• We identify the name of the under surface of an arch.
• Common related terms appear as options.

Concept / Approach:
The intrados is the inner, concave curve that forms the arch’s visible under surface. The term soffit is a general word for the underside of architectural members; in arches, soffit is often used synonymously with intrados, but 'intrados' is the specific technical term for the arch profile. The haunch is the part between crown and springing, and the back is the extrados or upper outer curve.

Step-by-Step Solution:
Match 'under surface of arch' → the inner curve = intrados.Recognize 'soffit' as a general underside term, but the arch-specific term sought is intrados.

Verification / Alternative check:
Standard texts define intrados as the inner curve; extrados (back) as the outer curve.

Why Other Options Are Wrong:

• Soffit: generic underside; less precise than intrados here.
• Haunch: zone of the arch, not the underside curve itself.
• Back: the outer (extrados) surface, not the inner.

Common Pitfalls:

• Using 'soffit' generically; exam questions typically prefer 'intrados' for specificity.

Final Answer:
intrados