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Investigation of polarized IR reflectance spectra of single crystals is important in the course of obtaining vibrational, optical and dielectric properties of solid state samples. Diffraction X-ray methods are usually employed for crystal orientation. In this work, two IR reflectance methods were proposed as a supplement in determining the orientation of the b crystal axis and by that the ac crystal plane, in respect to the morphology of monoclinic single crystals. The methods were tested on two Tutton salt single crystals (K2Ni(SO4)2·6H2O and K2Ni(SeO4)2·6H2O) which were further compared with the cross-polarization IR reflectance method. Both methods relied on the symmetry and optical properties of monoclinic single crystals. One takes into account the fact that one of the principal axes of the dielectric tensor was symmetry fixed, while the other used the conditions for the appearance of an isosbestic-like point/region in the polarized IR reflectance spectra of single crystals with low symmetry. [ABSTRACT FROM AUTHOR]