§ Of the Rings of Power and the Third Age The Silmarillion

Three Elendil took, and his sons each two. Those of Elendil were set in towers upon Emyn Beraid, and upon Amon Sûl, and in the city of Annúminas. But those of his sons were at Minas Ithil and Minas Anor, and at Orthanc and in Osgiliath. Now these Stones had this virtue that those who looked therein might perceive in them things far off, whether in place or in time. For the most part they revealed only things near to another kindred Stone, for the Stones each called to each; but those who possessed great strength of will and of mind might learn to direct their gaze whither they would. Thus the Númenóreans were aware of many things that their enemies wished to conceal, and little escaped their vigilance in the days of their might.