in·tra·dos /ˈɪntrəˌdɑs, ˌdo; ɪnˈtreˌdɑs/
穹隆之內面,拱之下麵,內弧面
In·tra·dos n. Arch. The interior curve of an arch; esp., the inner or lower curved face of the whole body of voussoirs taken together. See Extrados.
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intrados
n : the interior curve of an arch