Lamellar copper nanowires with a high density of traverse twins (twin spacing even less than one nm) are synthesized (see image). These wires spontaneously grow from a pure electrolyte of CuSO4 by electrodeposition. Micromechanical testing and electrical four-probe measurements impressively demonstrate that the highly twinned nanowires show a remarkably high mechanical strength while at the same time maintaining a high electrical conductivity.