Localized to the planar interface of two identical dielectric biaxial crystals with optic ray axes parallel to the interface but with a relative twist about an axis perpendicular to the interface, electromagnetic wave propagation along the bisectrix of the two crystallographic orientations is possible with either real-valued or complex-valued transverse decay constants as the twist angle varies from 0 degrees to an upper limit (< or =90 degrees) that is dependent on the angle between the two optic ray axes.