intrados

[ in-truh-dos, -dohs, in-trey-dos, -dohs ]

/ ˈɪn trəˌdɒs, -ˌdoʊs, ɪnˈtreɪ dɒs, -doʊs /

the interior curve or surface of an arch or vault.

Compare extrados.

Origin of intrados

1765–75; < French, equivalent to intra- intra- + dos back; see dosser¹

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
Intrados: The under or concave face of the solid structure of an arch.

How to judge architecture |Russell Sturgis
It was determined to attempt to stop these leaks by the application of a water-proof cement coating on the intrados of the arch.

Transactions of the American Society of Civil Engineers, Vol. LXVIII, Sept. 1910 |James H. Brace and Francis Mason
The under or concave side of the voussoirs is called the intrados, and the upper or convex side the extrados of the arch.

The New Gresham Encyclopedia. Vol. 1 Part 2 |Various

In S. Sophia the vault springs from the intrados of the transverse arches, that is, from the lower edge.

Byzantine Churches in Constantinople |Alexander Van Millingen

intrados

/ (ɪnˈtreɪdɒs) /

architect the inner curve or surface of an arch or vault Compare extrados

C18: from French, from intra- + dos back, from Latin dorsum