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Bytownite (a plagioclase feldspar with composition aboul An 80 Ab 20 ) closely resembles anorthite in its lattice dimensions and its diffracted intensities, except for the systematic absence of those wiih h + k odd ('c' and 'y' type). The deduction of a body-centred lattice (whence the previous name, 'body-centred anorthite') is here shown to be incorrect. Three-dimensional electron-density maps and difference maps show conspicuously elongated or doubled Ca/Na peaks and 'half-atom splitting' for other atoms. The results are explained in detail if bytownite has a primitive lattice and a structure very close to (but significantly different from) that of anorthite, and if small anti-phase domains are present with origins related by the vector ½(a + b + c). The evidence for this conclusion, and against other possible explanations of the observed facts, is discussed fully. Bond lengths and angles (tabulated in full) resemble those in anorthite, with some significant differences. Inequalities of bond lengths and angles within the same tetrahedron are real; an analysis of their resemblances and differences in corresponding tetrahedra in different feldspars suggeests an empirical approach to the study of the structural stress systems involved, and emphasizes the difference between the groups comprising microcline, reedmergnerite, and low albite on the one hand, and anorthite, bytownite, and high albite on the other. Ordering of Si/Al is nearly complete, on the same set of sites as in anorthite. At a closer approximation, a few Al-rich sites show significant differences in thier Si content, with extreme value of 0% and 30% against the average value of 13 ± 5% for the rest.