All of the following increase resistance and retention form on a SHORT clinical crown EXCEPT:

Correct answer: D. A chamfer finish line

• A. A PROXIMAL BOX serves the same function — it creates opposing vertical walls that resist rotation and lateral movement, and is a standard addition on short abutments.
• B. REDUCING the total occlusal CONVERGENCE toward the ideal 6-10 degrees is the single most effective way to improve both retention and resistance. As taper increases, both decline steeply; excessive taper is the most common preparation error.
• C. VERTICAL GROOVES placed in the axial walls are a primary means of adding resistance form to a short preparation. They restrict rotational displacement about the long axis and effectively lengthen the resisting wall surface.
• D. CORRECT — this is the exception. The CHAMFER is a FINISH LINE design. Its role is to define a clean, readable margin with adequate bulk of material and a smooth internal line angle for stress distribution. It contributes essentially nothing to resistance against lateral displacement, which is the specific problem on a short preparation.

Note: The relationship worth remembering: RETENTION resists displacement along the path of insertion and depends on near-parallel walls, surface area and cement. RESISTANCE resists displacement by oblique or lateral forces and depends on wall HEIGHT relative to width. On short preparations, height is the deficiency — so add grooves, boxes, or perform crown lengthening.