Polar surface induced asymmetric growth of single-side teethed ZnO nanocombs was attributed to the self-catalysis of the Zn-terminated (0001) surface (Z.L. Wang, X.Y. Kong, J.M. Zuo, Phys. Rev. Lett. 91 (2003) 185502). In this Letter, nanocombs of ZnO with double-sided teeth have been observed. This symmetric growth of the fish-ribbon like teeth has been identified due to the existence of an inversion domain boundary along the ribbon, so that both side surfaces of the ribbon are terminated with the chemically active Zn-(0001) plane. A model is also given about the formation of similar to 110 degrees double-sided nanocombs based on the nucleus composed of multiply twinned pyramids. The data show that the Zn-terminated (0 0 0 1) surface is responsible for the formation of the teeth, while the oxygen-terminated (000 (1) over bar) surface is chemically inactive and does not grow teeth. (c) 2005 Elsevier B.V. All rights reserved.