The literature on potash feldspar (including the modern textbooks) gives one the impression that, within the limits of observation, the several varieties of KAlSi3O8 have the same apparent lattice "symmetry" (α and γ of microcline being quoted as 90° within the limits of ±10′). Two principal opinions exist to explain the different modifications: (1) The atomic arrangement (structure) of all potash feldspar is the same, and this structure is triclinic. The differences in optical behavior are explained by differences in the size and shape of twinned portions in the material (Mallard's hypothesis). (2) Two different modifications of potash feldspar exist: triclinic microcline and monoclinic orthoclase. It has been suggested that this difference is due to a different degree of order-disorder between the Si and the Al atoms, triclinic microcline having a greater degree of order than monoclinic orthoclase (Barth's hypothesis). In this investigation it is shown that Mallard's hypothesis can be excluded, although...