The posterior border of the upper denture should extend 1-2 mm beyond the vibrating line to ensure an adequate seal and retention without impinging on movable soft tissues. The vibrating line represents the junction of the movable soft palate and the immovable hard palate, making it crucial for forming a posterior palatal seal. Extending slightly beyond this line compensates for functional movements, preventing food and air leakage under the denture while avoiding discomfort or gagging. This extension enhances denture stability by optimizing suction and preventing dislodgement during speech and swallowing.

Reference: Prosthodontics: Principles and Management Strategies, Stephen F. Rosenstiel, Edition 5.