Assembly of intermediate filaments by the addition of component monomers to a filament. Polymerization of intermediate filament proteins results from interactions among several distinct binding sites on the constituent proteins. Nuclear lamin head-to-tail polymers arise from one such interaction. Deletion analysis localized the binding sites to the ends of the rod domain that are highly conserved among all intermediate filament proteins. Data indicate that one type of interaction in intermediate filament protein polymerization is the longitudinal binding of dimers via the conserved end segments of the coiled-coil rod domain.