A regular three-dimensional structure can be formed from repeating subunits using either helical symmetry or icosahedral symmetry principles. In the case of the smallest viruses, the final structure is simple and quite regular. Larger viruses with
more proteins at their disposal can build more elaborate structures. Enveloped viruses may be quite regular in construction or may have irregular features, because the use of lipid envelopes allows irregularities in construction.