  Nanotubes show great promise for miniaturizing advanced technologies. Their
exceptional physical properties are intimately related to their morphological
and crystal structure. Circumferential faceting of multiwalled nanotubes
reinforces their mechanical strength and alters their tribological and
electronic properties. Here, the nature of this important phenomenon is fully
rationalized in terms of interlayer registry patterns. Regardless of the
nanotube identity (that is, diameter, chirality, chemical composition),
faceting requires the matching of the chiral angles of adjacent layers. Above a
critical diameter that corresponds well with experimental results, achiral
multiwalled nanotubes display evenly spaced extended axial facets whose number
equals the interlayer difference in circumferential unit cells. Elongated
helical facets, commonly observed in experiment, appear in nanotubes that
exhibit small interlayer chiral angle mismatch. When the wall chiralities are
uncorrelated, faceting is suppressed and outer layer corrugation, which is
induced by the moir\'e superlattice, is obtained in agreement with experiments.
Finally, we offer an explanation for the higher incidence of faceting in
multiwalled boron nitride nanotubes with respect to their carbon-based
counterparts.
