An existing model [L. Bellarosa, E. Bakalis, M. Melle-Franco, F. Zerbetto, Nano Lett. 6 (2006) 1950] predicts that the structures of the seven double-wall carbon nanotubes, DWCNTs, identified by high-resolution transmission electron microscopy [L. Guan, K. Suenaga, S. Iijima, Nano Lett. 8 (2008) 459] are the most stable. Since the samples were obtained from single-wall carbon nanotubes, SWCNTs, filled with ferrocene molecules that were annealed at 1273 K for 24 h, it is concluded that under proper conditions the host SWCNT patterns the formation of the inner one to form the most stable pair. (c) 2008 Elsevier B. V. All rights reserved.