A lingual plate is indicated when there is a high lingual frenum because it provides additional support and stabilization to the denture by extending superiorly to accommodate and relieve tension from the frenum. This design prevents displacement or lifting of the prosthesis during tongue movements caused by the high frenum. The plate also ensures preservation of the floor of the mouth space, maintaining comfort and retention. In contrast, other conditions like low floor of the mouth or shallow vestibule require different design modifications.

Reference: Prosthodontics, Rosenstiel, Edition 6.