soffit

[ sof-it ]

/ ˈsɒf ɪt /

the underside of an architectural feature, as a beam, arch, ceiling, vault, or cornice.

Origin of soffit

1605–15; < French soffite < Italian soffitto < Vulgar Latin *suffīctus, for Latin suffīxus; see suffix

The soffit is flat, with a round moulding at the inner and the outer edge.

The Story of Chartres |Cecil Headlam
Flat arch, where the soffit is horizontal and sometimes slightly cambered (dotted line).

Encyclopaedia Britannica, 11th Edition, Volume 2, Slice 4 |Various
The soffit of the arch is carved, and the face of the pilaster below has a very rich and graceful vine arabesque upon it.

The Shores of the Adriatic |F. Hamilton Jackson
A projection upon the soffit of the Doric corona, which originally marked the position of the rafter-ends beneath the sheathing.

History of Ancient Art |Franz von Reber

The lower surface or soffit of the arch is the intrados, E, and the upper surface the extrados, F.

Encyclopaedia Britannica, 11th Edition, Volume 2, Slice 4 |Various

soffit

/ (ˈsɒfɪt) /

the underside of a part of a building or a structural component, such as an arch, beam, stair, etc

Also called: crown, vertex the upper inner surface of a drain or sewer Compare invert (def. 6)

C17: via French from Italian soffitto, from Latin suffixus something fixed underneath, from suffīgere, from sub- under + fīgere to fasten