 The basic tactical unit was the "éored", Old English for "a unit of cavalry, a troop", which at the time of the War of the Ring had a nominal strength of 120 riders.In time of war, every able man was obliged to join the Muster of Rohan.They were also bound by the Oath of Éorl to help Gondor in times of peril, and the latter asked for their aid through the giving of the Red Arrow.