Chirality
For a given (n,m) nanotube, if n - m=3q (where q is an integer), then the nanotube is metallic, otherwise the nanotube is a semiconductor. Thus all armchair nanotubes (n=m) are metallic, and nanotubes (5,0), (6,4), (9,1), etc. are semiconducting.
Read more about this topic: Fermi Point