The superior border of a lingual bar must be positioned at least 4 mm below the gingival margin to ensure adequate biological width is maintained. This space prevents impingement on the gingival tissues, reducing the risk of inflammation and allowing sufficient room for the lingual vestibule and soft tissue thickness. Additionally, this positioning avoids trauma to the marginal gingiva during function and facilitates effective oral hygiene. Insufficient clearance can cause discomfort and compromise the longevity of the prosthesis.

Reference: Prosthodontics, Anil Singh, 2nd Edition.