From Tolkien Gateway
The Master-stone was the chief of all the palantíri, both in Aman and Middle-earth, which was held in the Tower of Avallónë on the Lonely Isle. It was said that the Seeing-stone in Elostirion in the Tower Hills could be used to look back along the Straight Road and glimpse the Tower.
The Stone of Avallónë is variously named the "'Masterstone'" or Master-stone. There was also a 'Master Stone' of Middle-earth, the palantír of Osgiliath, but this was a lesser Stone than the Masterstone in Avallónë, having power only over other palantíri in Middle-earth.
|Stones of Arnor||† Annúminas-stone (Annúminas) · Elostirion-stone (Elostirion - Valinor) · † Amon Sûl-stone (Amon Sûl - Fornost)|
|Stones of Gondor||† Osgiliath-stone (Osgiliath) · † Ithil-stone (Minas Ithil - Barad-dûr) · Anor-stone (Minas Tirith) · Orthanc-stone (Orthanc - Minas Tirith)|
|Other||Master-stone (Tower of Avallonë)|