(color online) Structures of ZW-CNTs based on the tubes (6, 6) and (20, 20), i.e., (a) ZW₁-(6, 6), (b) ZW₂-(6, 6), (c) ZW₄-(6, 6), and (d) ZW₈-(20, 20). The integer N refers to the number of carbon atoms in the primitive cell. For comparison, the red balls show these atoms bonding to four nearest-neighbor atoms by the sp³ hybrid orbits.