in a certain system of Cartesian coordinates. This reflects the space along an (m − k) -dimensional affine subspace. If k = m, then such a transformation is known as a point reflection, or an inversion through a point. On the plane (m = 2), a point reflection is the same as a half- turn (180°) rotation; see below. Antipodal symmetry is an alternative name for a point reflection symmetry through the origin.