The name used only for military organization in the West of Rohan. The West-mark included all of the lands west of the Entwash and Snowbourn rivers, except those lands in the immediate vicinity of Edoras. There, a Marshal of the Mark was the chief military commander and he typically made his home base at Helms Deep. For a time during the War of the Ring, Théodred son of Théoden served as the Second Marshal of the Mark, based in the West-mark.

The West-mark had long been an area greatly troubled by the enemies of Rohan as it lay on the borders of Dunland and also of Isengard. It was during the First Battle at the Fords of Isen with the army of Saruman that Théodred lost his life. After Théodred died, Erkenbrand took over the West-mark and after fighting and losing a second battle at the Fords of Isen, he came to the aid of the Rohirrim at Helms Deep.
Encyclopedia entry originally written by Nienna-of-the-Valar