Summary: The position of the 2¯01 X-ray reflection in albite varies with the degree of Al/Si ordering. This is also true for binary plagioclases but the 2¯01 reflection for high-temperature plagioclases is constant and therefore independent of anorthite content. For plagioclases of ternary composition grown in a hydrous silicate melt, the position of the 2¯01 reflection gives a good estimate of the orthoclase content when a determinative curve is used that was originally drawn from data on high-temperature synthetic binary alkali feldspars.