  The dispersion relations of carbon nanotubes are often obtained
cross-sectioning those of graphene (zone-folding technique) in a rectangular
region of the reciprocal space, where it is easier to fold the resulting
relations into the nanotube Brillouin zone. We propose a particular choice of
the unit vectors for the graphene lattice, which consists of the symmetry
vector and the translational vector of the considered carbon nanotube. Due to
the properties of the corresponding unit vectors in the reciprocal space, this
choice is particularly useful for understanding the relationship between the
rectangular region where the folding procedure is most easily applied and the
overall graphene reciprocal space. Such a choice allows one to find, from any
graphene wave vector, the equivalent one inside the rectangular region in a
computationally inexpensive way. As an example, we show how the use of these
unit vectors makes it easy to limit the computation to the bands nearest to the
energy maxima and minima when determining the nanotube dispersion relations
from those of graphene with the zone-folding technique.
