For more than 300 years there has been evidence for relationships between the radii of planetary orbits in the solar system, known since about 1766 as the Titius-Bode Law, (TBL). According to this law, the radii increase exponentially in equidistant steps. Due to recent findings, substantial modifications of this law seem to be appropriate: 1). Not the radii, but the period lengths are correlated.2). Resonant ratios of the period durations determine the behavior.3). In more and more exosolar planetary systems and planet-moon systems this behavior is registered. 4). Which unknown coupling mechanism causes this resonance?Neither in its original form, nor in its "generalized" form, TBL can be explained on the basis of classical gravitational theories. On the basis of ARG, TBL may very well be expected, with all the details given above. Thus TBL, older than "Mach's Priciple" and "Foucault Pendulum", may well be counted among the effects which are particularly well suited as a litmus test for all theories of gravitation.