In the absence of anisotropy, the observed polarization of shear waves should be those radiated from the source, modified only by interaction with internal interfaces and free surface topography. It has been suggested that shear wave polarizations may be systematically distorted by internal interfaces, which could lead to difficulties in interpreting anisotropy-induced shear wave splitting. Since the literature now reports many investigations of shear wave splitting, possible complications caused by internal interfaces would be important and need to be assessed. In this paper, we investigate the polarization of shear wave in full-wave synthetic seismograms incident from a point source at an isotropic-to-isotropic interface, over a range of angles of incidence. Particle motion due to plane and curved wave fronts has different characteristics. Internal shear wave windows can be defined, analogous to the shear wave window at the free surface. Within the innermost window, the polarizations at subsurface geophones (as in vertical seismic profiles) are recorded with little distortion, but outside this window the waveforms become increasingly distorted for large⋅angles of incidence. This is similar to the behavior of shear waves with respect to the shear wave window at the free surface, except that the behavior at internal interfaces is more complicated because of additional critical angles, and additional windows. We conclude that the behavior at internal interfaces is more complicated because of additional critical angles, and additional windows. We conclude that the interaction of shear waves with internal interfaces is only a serious difficulty when interpretating wide offset vertical seismic profiles, wide angle reflections, and cross-hole data. ¿American Geophysical Union 1990