(1) the essentially vertical extension from the body of the denture into one of the vestibules of the oral cavity; also, on the lower denture, the essentially vertical extension along the lingual side of the alveololingual sulcus; (2) the buccal and labial vertical extension of the upper or lower denture base, and the lingual vertical extension of the lower one; the buccal and labial denture flanges have two surfaces: the buccal or labial surface and the basal seat surface; the lower lingual flange also has two surfaces: the basal seat surface and the lingual surface.