This letter addresses the direct effect that geometry has in controlling the mechanisms of inelastic deformation in metal nanowires. By performing atomistic simulations of the tensile deformation of < 100 >/{100} hollow copper nanowires (nanoboxes), the authors find that the nanoboxes deform in an unexpected twinning-dominated mode; the nonsquare wall geometries of the nanoboxes bias the deformation by allowing the larger transverse {100} surfaces to reduce their area through twinning by reorienting to a lower energy {111} surface. Additional analyses on solid nanowires with nonsquare cross sections confirm that geometry can be utilized to engineer the mechanical behavior and properties of nanomaterials. (c) 2006 American Institute of Physics.