Stereograms are classified into those with constant disparity and those with continuously variable disparity. The first class gives rise to the perception of a displaced flat plane, the second gives rise to the perception of a tilting surface. The former class includes most of the stereograms studied to date: the latter includes those derived from considerations of aniseikonia. Certain ramifications of this classification arc noted. as well as the limits it sets on possible brain models of binocular stereoscopic function.