The melt structures of linear polyethylene and the isotactic vinyl polymers polypropylene, poly(1-butene), and poly(4-methyl-1-pentene), along with the corresponding methyl, ethyl, and isobutyl side chains, were studied with wide-angle X-ray diffraction. As the size of the side branch increases from zero (polyethylene) to methyl, ethyl, and isobutyl, a prepeak appears below the main diffraction peak in the total structure factor. The prepeaks become stronger and shift to lower scattering vectors with increasing bulkiness of the side chain. There is a strong correlation between the position of the prepeaks in the melt and the average nearest-neighbor helix-helix packing distance in the crystals, implying similar helical conformations in the melts.