Summary: The position of the rhombic section of plagioclase felspars, as calculated from the measured cell dimensions, has been shown to vary with the structural state, the variation from high- to low-temperature forms being most pronounced for albite. The theoretical composition plane of pericline twins is the rhombic section. If the structural state or the composition of the felspar changes after twinning has occurred, the cell dimensions and hence the rhombic section will change; however, the composition plane of the pericline twin will probably remain unchanged unless recrystallization occurs, or perhaps if the felspar is subjected to strong stress. Thus the observed pericline composition plane need not coincide with the rhombic section. Comparison of observed composition planes with the corresponding calculated rhombie sections shows that some of them do indeed differ and the differences have been used to provide information on the changes occurring subsequent to the establishment of the pericline twins. The interpretation is complicated by the occurrence of the acline-A twin, whose composition plane is {001}, for the rhombie section of plagioclase felspars from An₀ to An₆₀ may coincide with {001} if the structural state of inversion has a particular value. When the observed composition would agree with both pericline and acline-A twinning, the new term b-axis twin has been used.