INTRADOS
Source: 566, 567
intrados. Intrados, n. the interior side of an arch
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In‐tra″dos (?), n. [[F., fr. L. intra within + F. dos the back, L. dorsum. Cf. Extrados.]] (Arch.) The interior curve of an arch; esp., the inner or lower curved face of the whole body of voussoirs taken together. See Extrados.