We show that, and how, window coordinate precision (the representations of xwin and ywin), field of view, and error accumulated by single-precision mapping arithmetic contribute to, and sometimes dominate, effective z-buffer resolution. Our results are developed analytically, then verified through simulation. Using our approach system designers can allocate numeric precision more efficiently, and programmers can more confidently predict the minimum triangle-to-triangle separation required to ensure correct z-buffer occlusion.