The article explores the phenomenon of wandering principal optical axes in van der Waals (vdW) materials with low-symmetry triclinic structures, specifically rhenium disulfide (ReS₂) and rhenium diselenide (ReSe₂). These materials exhibit anisotropic permittivity tensors, which are typically assumed to be stationary. However, the study reveals that the principal optical axes can rotate by more than π/2 degrees in the space-wavelength domain, leading to wavelength-switchable propagation directions of waveguide modes. This effect is attributed to the non-orthogonal exciton resonances in these materials, which are a result of their reduced symmetry. The physical origin of this phenomenon is explained using a multi-exciton phenomenological model and ab initio calculations. The authors demonstrate that the wandering principal optical axes offer a platform for exploring unexplored anisotropic phenomena and nanophotonic applications, such as wavelength-switchable optics and metamaterials. The findings highlight the importance of symmetry in determining the optical properties of materials and suggest that similar effects may be observed in other low-symmetry vdW crystals.The article explores the phenomenon of wandering principal optical axes in van der Waals (vdW) materials with low-symmetry triclinic structures, specifically rhenium disulfide (ReS₂) and rhenium diselenide (ReSe₂). These materials exhibit anisotropic permittivity tensors, which are typically assumed to be stationary. However, the study reveals that the principal optical axes can rotate by more than π/2 degrees in the space-wavelength domain, leading to wavelength-switchable propagation directions of waveguide modes. This effect is attributed to the non-orthogonal exciton resonances in these materials, which are a result of their reduced symmetry. The physical origin of this phenomenon is explained using a multi-exciton phenomenological model and ab initio calculations. The authors demonstrate that the wandering principal optical axes offer a platform for exploring unexplored anisotropic phenomena and nanophotonic applications, such as wavelength-switchable optics and metamaterials. The findings highlight the importance of symmetry in determining the optical properties of materials and suggest that similar effects may be observed in other low-symmetry vdW crystals.