The origin of the ferroelectric domain contrast on triglycine sulfate (TGS) and guanidinium aluminum sulfate hexahydrate (GASH) observed in lateral force microscopy (LFM) is presented. On the TGS (010) cleavage face, four different friction coefficients are found due to the different chemical composition of oppositely polarized domains and to asymmetric surface potentials. Moreover, these friction coefficients depend upon the orientation of the crystallographic lattice relative to the scan direction. On the GASH (0001) cleavage face, structural differences between the surfaces of oppositely polarized domains modify the surface potential experienced by the LFM tip, resulting in two different friction coefficients. Additionally, it could be shown that electrostatic interactions between tip and sample have no detectable influence on the measured torsion of the cantilever.