An Éored was a division of the Rohirrim consisting of a considerable body of men, fully trained for war, who might serve for a specific term or sometimes as a permanent group. In the early history of Rohan the number could vary, but in the days of King Folcwine a "full Éored" was defined as 120 men (including its captain) and was one hundredth part of the Full Muster of the Riders of the Mark. In times of war each Marshal of the Mark had as part of his "household" an Éored ready for battle that he could use at his own discretion. If the king were riding to war his Éored was called "the King's Company".
In T.A. 1899 when King Calimehtar of Gondor fought the Wainriders his cavalry joined with a great Éored led by Marhwini to attack the enemy in the flank and rear (this was when an Éored did not have a strictly determined size).
In T.A. 3019 the Éored that Éomer used to pursue the Orcs to the border of Fangorn Forest had 120 men - Legolas counted 105 and Éomer stated that 15 men had been lost (and 12 horses). Later, when King Théoden rode to Minas Tirith, Elfhelm commanded the Éored in which Dernhelm (and Merry) rode.
The word "Éored" actually was used by the Anglo-Saxons to denote a division.
- J.R.R. Tolkien, Christopher Tolkien (ed.), Unfinished Tales, "Cirion and Eorl and the Friendship of Gondor and Rohan", note 36
- J.R.R. Tolkien, Christopher Tolkien (ed.), Unfinished Tales, "The Battles of the Fords of Isen", Appendix (ii)
- J.R.R. Tolkien, The Lord of the Rings, Appendix B, "The Third Age"
- J.R.R. Tolkien, Christopher Tolkien (ed.), Unfinished Tales, "Cirion and Eorl and the Friendship of Gondor and Rohan", The Northmen and the Wainriders
- J.R.R. Tolkien, The Lord of the Rings, The Two Towers, "The Riders of Rohan"
- J.R.R. Tolkien, The Lord of the Rings, The Return of the King, "The Ride of the Rohirrim"