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In their monumental structures in stone, the arch was sometimes built with regular voussoirs, i.e. with a semicircular extrados, and sometimes with the joint carried far beyond.

The under or concave side of the voussoirs is called the intrados, and the upper or convex side the extrados of the arch.

The form of the mound may be described as reproducing the extrados of a depressed arch.

"Are you still wondering what the extrados of a voussoir is?"

The four sides of the mold formed the extrados, the intrados, and the two ends of the block; the other two sides being left open.