Anisotropic acousto-optic diffraction in crystals is the fundamental phenomenon that is used to design acousto-optic tunable filters. Noncritical and quasicollinear phase-matching geometries of Bragg acousto-optic diffraction are compared for acoustic symmetry planes in four crystal systems (tetragonal, trigonal, orthorhombic, and monoclinic). The results for uniaxial crystals are reviewed and generalized for biaxial crystals. It is shown that cubic frequency dependence on the Bragg angle exists in two symmetry planes of orthorhombic crystals and conditionally exists in the symmetry plane of monoclinic crystals. It is also shown that there are two points in the symmetry plane of monoclinic crystals where noncritical phase matching takes place in quasicollinear diffraction geometry that can be used to design high-resolution tunable filters. Phase-matching configurations in α-iodic acid and potassium gadolinium tungstate crystals are analyzed.