  Some X-shaped radio galaxies (XRGs) show a Z-symmetric morphology in the less
luminous secondary lobes. Our geometrical arguments strongly support a merger
of two galaxies as mechanism for the formation of these sources (ZRG). They
also strengthen the conjecture that a jet is aligned with the spin of the BH at
its base and that the jet flips into the direction of the orbital angular
momentum of the pre-merger binary black hole (BHB). We could also restrict the
distance where the pre-merger jet is bent into Z-shape by the inspiralling
galaxy to the range of 30-100 kpc. One of three possible orientations of the
jet relative to our line of sight is more likely than the others and allows us
to deduce the direction of the spin of the merged BH. The existence of XRGs and
ZRGs proves that the binary has merged, contrary to previous speculations that
after a merger of two galaxies the decay of the BHB stalls due to loss cone
depletion. In ZRGs the black holes probably merge on timescales of some 10^8 yr
after the bending of the jet in a distance of about 50 kpc. Thus, in a way, the
bending starts a stop watch for the rest of the merger.
