decagon

To construct a regular pentagon, we find it convenient first to construct a regular decagon.

Below the arches are the chapels which surround the decagon in symmetrical fashion.

The plan of the church was a decagon, within which six composite columns in the centre supported six semi-circular vaults.

In the fourth are six triangles, in the fifth, five polygons from a pentagon to a decagon.

Decagon, dek′a-gon, n. a plane figure of ten angles and sides.