The buccal and lingual walls are smoothly connected to the occlusal isthmus. An "s-curve" is used only when needed to preserve tooth structure in providing this smooth-flowing continuous outline form without sharp or abrupt turns. Note the preservation of the buccal cusps as s-curves are used to circumscribe them as the buccal walls of the box and occlusal portions are connected. While the lingual cusps are likewise preserved, s-curves are not needed in these preparations.
A: Excessive s-curve (90 degree turns) - Make curve more gentle
B: Unsupported enamel rods at cavosurface - Plane proximal walls perpendicular to the cavosurface
C: Sharp turns - Blend proximal walls into occlusal isthmus with gentle s-curves
D: Flared proximal walls with acute amalgam angles at cavosurface - Make proximal walls perpendicular to cavosurface blending with s-curve to isthmus
E: Beveling at cavosurface - Correct as in D above
