The margin of a tooth preparation should end at least 2 mm from the gingival crest to preserve the biological width and maintain healthy periodontal tissue. This distance allows adequate space for the junctional epithelium and connective tissue attachment, preventing inflammation and attachment loss. Placing margins too close to the gingiva (<2 mm) can lead to chronic irritation, gingival recession, and bone resorption. Maintaining this clearance supports optimal healing and long-term tooth stability after restoration.

Reference: Fundamentals of Operative Dentistry, 4th Edition.