Old Gondor

When Gondor was founded, it was ruled jointly by the brothers Isildur and Anárion. Each of these dwelt in their own city, but the capital of the entire realm was Osgiliath on Anduin, where the thrones of the sons of Elendil stood side by side in the Great Hall. After the loss of the brothers in the War of the Last Alliance and its aftermath, Meneldil and the line of Kings who followed him maintained Osgiliath as the chief city of their realm.

The arrangement lasted for more than four centuries, until the time of King Ostoher. He rebuilt Anárion's city of Minas Anor, and though he maintained Osgiliath as his capital, from his time forward it was the custom of the Kings to spend summers in Minas Anor and winters in Osgiliath.

This custom lasted for the next thousand years, but in III 1437 the civil war of the Kin-strife saw Osgiliath burned, and the Great Plague two centuries later left parts of the city deserted. At this time King Tarondor chose to remove the royal seat permanently to Minas Anor. Left near-abandoned, Osgiliath's fall was long and slow, but in III 2475 it was invaded by Uruks out of Mordor, and though they were driven back, the fighting in the city left it completely ruined.

Thus Osgiliath came to be called 'Old Gondor', a memory of a great realm that was now broken and lost, distantly visible from the walls of Minas Tirith (as Minas Anor had become known) over the five centuries that led up to the War of the Ring.