The structural and electronic properties of sheets and nanotubes of boron are investi-ated using density functional theory. The calculations predict the stability of a novel reconstructed {1221} sheet over the 'idealized' triangular {1212} sheet. Nanotubes formed by wrapping the half-metallic {1221} sheet show a curvature-induced transition in their electronic properties. Analysis of the charge density reveals a mixed metallic- and covalent-type of bonding in the reconstructed {1221} sheet and the corresponding nanotubes, in contrast to metallic-type bonding in the idealized It {1212} sheet and its analogous nanotubes. (c) 2005 Elsevier B.V. All rights reserved.