  Many asteroids in the main and trans-Neptunian belts are trapped in mean
motion resonances with Jupiter and Neptune, respectively. As a side effect,
they experience accidental commensurabilities among themselves. These
commensurabilities define characteristic patterns that can be used to trace the
source of the observed resonant behaviour. Here, we explore systematically the
existence of commensurabilities between the known ETNOs using their
heliocentric and barycentric semimajor axes, their uncertainties, and Monte
Carlo techniques. We find that the commensurability patterns present in the
known ETNO population resemble those found in the main and trans-Neptunian
belts. Although based on small number statistics, such patterns can only be
properly explained if most, if not all, of the known ETNOs are subjected to the
resonant gravitational perturbations of yet undetected trans-Plutonian planets.
We show explicitly that some of the statistically significant
commensurabilities are compatible with the Planet Nine hypothesis; in
particular, a number of objects may be trapped in the 5:3 and 3:1 mean motion
resonances with a putative Planet Nine with semimajor axis of about 700 au.
