ex·tra·dos /ˈɛkstrəˌdɑs, ˌdo; ɛkˈstreˌdɑs/ 拱背;拱洞的外弧面
Ex·tra·dos n. Arch. The exterior curve of an arch; esp., the upper curved face of the whole body of voussoirs. See Intrados. ◄ ►
extrados n : the exterior curve of an arch