To see this, imagine an isosceles triangle with a altitude drawn.
Being either isosceles or scalene is inseparable from a triangle in real existence.
As the Greek philanderer isosceles used to say, " There are 3 sides to every triangle " .
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.
The demihexagon is an isosceles trapezium (trapezoid in American usage) in which three of the sides have the same length.