The disposition of three of the four Stones of Gondor before the loss of the Master Stone in III 1437. (The fourth of Gondor's Stones was held in Orthanc, far to the north and west of the area shown here.)

The great Seeing-stone that stood beneath the Dome of Stars in Osgiliath. It was the chief of the palantíri of Middle-earth, and had power over the other Stones (it is known, for example, that it had the unique ability to observe conversations between the users of other palantíri). When Osgiliath was besieged by Castamir in III 1437, the Tower that held the Stone of Osgiliath was broken, and the Master Stone was lost in the Anduin.

There is some confusion over the term 'Master Stone', because there are references to another palantír variously known as the 'Masterstone' or 'Master-stone', held in the Tower of Avallónë on Tol Eressëa. The implication of this is uncertain, but it appears that the Master Stone of Osgiliath had power only over the other six palantíri of Middle-earth, and not those that still existed in the West.