  We investigate the mechanism of polar alignment for accretion discs in
hierarchical systems (HSs) with more than two stars. In eccentric binary
systems, low mass discs that are sufficiently tilted to the binary orbit align
in a polar configuration with respect to the binary plane by aligning their
angular momentum to the binary eccentricity vector. In HSs, secular evolution
of the orbital parameters makes the eccentricity vector of the system precess
with time. This precession undermines the stability of the polar orbit for
accretion discs hosted in HSs. We analytically show that the binary criteria
for polar alignment derived in the literature are necessary but not sufficient
conditions for polar alignment in HSs. Then, we derive an analytical criterion
for polar alignment in HSs. In general, we find that discs orbiting the
innermost level of a HS can go polar. Conversely, radially extended discs
orbiting the outer levels of a HS cannot polarly align and evolve as orbiting
around a circular binary. We confirm our findings through detailed numerical
simulations. Also, our results are compatible with the observed distribution of
disc-orbit mutual inclination. Finally, we compare the observed distribution of
disc inclinations in the binary and in the HS populations. Binaries host mainly
coplanar discs, while HSs show a wide range of disc inclinations. We suggest
that the wider range of inclinations in HSs results from the secular
oscillation of their orbital parameters (such as Kozai-Lidov oscillations),
rather than from a different initial condition or evolution between HSs and
binaries.
