Oath of Eorl
The Oath of Eorl is the alliance sworn between the Middle-earth nations of Rohan and Gondor. The Oath also stated that the province of Calenardhon will be forever of Rohan, and the borders of that kingdom were established.
Also agreed was whenever help was needed, the two kingdoms would aid each other. The signal that help was needed was the Red Arrow (which means danger). This arrow was brought to King Théoden of Rohan by a messenger of Gondor in The Return of the King while Minas Tirith was besieged by Sauron. For faster communication between the two allies, the warning beacons of Gondor were used. These beacons were placed on seven hills along the northern ridge of the White Mountains.