The number of different orientations of axes that have been employed for the description of orthorhombic crystals is really quite remarkable and has resulted in an unnecessary variation in the labeling of the symmetry species of modes observed for this class of crystals. Results of factor group analysis of three different groups of workers are collected in Table I to illustrate this problem. Direct comparison of results is unnecessarily complicated. Similar ambiguities exist for crystals with the aragonite structure, e.g., CaCO<sub>3</sub>, and KNO<sub>3</sub>, and can arise for any orthorhombic crystal.
M. H. Brooker, "Recommendations for Factor Group Analysis of Orthorhombic Crystals: Application to K2SO4," Appl. Spectrosc. 29, 528-530 (1975)