This is fulfilled when the opposite sides of the hexagon are parallel, and (as a still more special case) when the hexagon is regular.
The town cross is a fine structure standing upon a huge hexagon, surmounted by a stone pillar 12 ft.
Consider, for example, a frame whose sides form the six sides of a hexagon ABCDEF and the three diagonals AD, BE, CF; and suppose that it is required to find the stress in CF due to a given system of extraneous forces in equilibrium, acting on the joints.
The octahedral formula discussed by Julius Thomsen (Ber., 1886, 19, p. 2 944) consists of the six carbon atoms placed at the corners of a regular octahedron, and connected together by the full lines as shown in (I); a plane projection gives a hexagon with diagonals (II).
I don't think it was designed to be an irregular hexagon in plan.