Arch. [a. F. extrados, f. L. extrā outside + F. dos the back.] The upper or exterior curve of an arch; esp. the upper curve of the voussoirs or stones that immediately form the arch. Cf. INTRADOS.

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1772.  Hutton, Bridges, p. iii. The relations between their intrados and extrados.

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1823.  P. Nicholson, Pract. Build., 338. Extrados of a Bridge—The curve of the road-way.

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1828.  Hutton, Course Math., II. 172. So that the extrados is a parabola equal to the intrados, and everywhere vertically equidistant from it.

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1879.  Sir G. Scott, Lect. Archit., II. 141. We have … supposed our arches to be of moderate depth from extrados, or outer line, to intrados or inner line.

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