It appears not to be widely known that there is an analogy of the theorem of Pythagoras for a ‘right-angled tetrahedron’, i.e. a tetrahedron which can be formed by slicing off a vertex from a cuboid. The result is: the square of the area of the face opposite the vertex containing the three 90° dihedral angles is equal to the sum of the squares of the areas of the other three faces.