  A simple model for the nucleation and growth of single wall carbon nanotubes
from a graphene sheet at the surface of a metallic catalyst saturated in carbon
is developed. It enables to predict the geometry and energy of tube embryos of
all possible chiralities, as well as the way that they can grow. It is shown
that armchair-like chiralities are energy preferred for geometrical reasons.
This result is discussed and compared to experimental literature.
