A longitudinal ferroelectric smectic phase lacks a reflection or mirror symmetry, which may be achieved with appropriate chemical design of molecules. Using x-ray diffraction, we have examined three candidate systems: a fraternal-twin triester, a mixture of polyphilic compounds, and a cyclic siloxane co-oligomer. These systems have been reported to exhibit properties indicative of longitudinal ferroelectric phases. A comparative structural analysis of these materials is presented.