  The physics of Josephson tunnel junctions drastically depends on their
geometrical configurations. The shape of the junction determines the specific
form of the magnetic-field dependence of the its Josephson current. Here we
address the magnetic diffraction patterns of specially shaped planar Josephson
tunnel junctions in the presence of an in-plane magnetic field of arbitrary
orientations. We focus on a wide ensemble of junctions whose shape is invariant
under point reflection. We analyze the implications of this type of isometry
and derive the threshold curves of junctions whose shape is the union or the
relative complement of two point symmetric plane figures.
