Imagine a perfect cube that is placed on a horizontal surface resting on its point (A), so that the opposite point (B is in the air directly above it. (The long diagonal of this cube (AB) would thus be perpendicular to the surface.)

The cube is then cut in half with a horizontal cut - parallel to the surface it is standing on, at the height equal half the length of AB, resulting in two equal shapes.

What is the cross-section of such a cut?