When hardened in situ its shape is that of a right-angled, triangular prism showing five surfaces - superior, anterior, inferior, posterior and right lateral which represents the base of the prism.

Take any two arbitrary directions in the plane of the paper, and draw a small isosceles triangle abc, whose sides are perpendicular to the two directions, and consider the equilibrium of a small triangular prism of fluid, of which the triangle is the cross section.