Tooth preparation must terminate at least 2 mm from the gingival crest to preserve the biologic width, which includes the junctional epithelium and connective tissue attachment. Violating this space can cause inflammation, gingival recession, and attachment loss. Maintaining a 2 mm distance ensures adequate space for the restoration margin without compromising periodontal health. This principle is critical for successful restorative outcomes and long-term tooth prognosis.

Reference: Principles of Operative Dentistry, STANKS, 4th Edition.