What's the difference between extrados and intrados?

Extrados

Definition:

• (n.) The exterior curve of an arch; esp., the upper curved face of the whole body of voussoirs. See Intrados.

Example Sentences:

Intrados

Definition:

• (n.) The interior curve of an arch; esp., the inner or lower curved face of the whole body of voussoirs taken together. See Extrados.

Example Sentences: