High-resolution electron microscopy has been used to study the regularities of formation of symmetric helical carbon filaments from Ni-Cu/MgO-catalyzed butadiene-1,3 decomposition at 450 degrees C. Dissymetrical spirally twisted carbon filaments have been shown to grow from a single Ni-Cn alloy particle. As the filaments grow, a microphase of metastable nickel carbide Ni3C forms on the particles' frontal side, whereas the crystal particle of Ni-Cu alloy possesses a series of twinning planes (111) that is parallel to the dissymmetry plane of the crystal and separates the blocks of twins with a face-centered structure. The mechanism of symmetric helical filamentous carbon growth has been suggested.