Published chemical analyses demonstrate that the nepheline formula should be written K_xNa_yCa_z□_s-(x+y+z)Al_x+y+2zSiSi_16-(x+y+2z)OO₃₂, where □ stands for vacant sites. X-ray data are presented for the nepheline phase in four binary systems: Ne-CaAl₂O₄, Ne-An, Ne-Ab, Ne-Kp. Only in two of these systems do the cell-dimensions change with composition. In the first one, the cell-volume V increases linearly with increasing calcium content; in the last one, two singularities in the curve of V against x divide the phase Na_8-xK_xAl₈Si₈O₃₂ into three subphases: subpotassic (0 < x < 0·25), mediopotassic (0·25 < x < 2·00), and perpotassic (2·00 < x < 4·73). Only in the subpotassic range are both high- and low-temperature forms found. Twenty-eight natural nephelines, for which chemical analyses and X-ray data are available in the literature, show that only the potassium content affects cell-dimensions. Although all analysed natural nephelines fall outside the subpotassic range, the re-examination of a Monte Somma specimen studied by Bannister (1931) reveals a few euhedral crystals of subpotassic nepheline in a mediopotassic phase.