The Gap of Rohan, once known as Gap of Calenardhon was the opening between the mountain ranges of the Misty Mountains and the White Mountains.
Through the Gap flowed the river Isen, which formed the effective western boundary of the region Calenardhon, later Kingdom of Rohan. The ancient North-South Road which connected the realms of Gondor and Arnor of old ran through the Gap, crossing the Isen at the Fords of Isen. Access to the Gap was controlled by the fortress of Angrenost, or Isengard, which in the later Third Age was controlled by Saruman.
The triangle between the Isen and Adorn rivers and the White Mountains was a contested area between the Rohirrim and the Dunlendings.
Both Boromir and the Black Riders passed westward through the Gap in T.A. 3018.
During the War of the Ring, the Fellowship of the Ring chose not to travel through the gap because they had learned Saruman was in league with Sauron, and instead passed through Moria.
The Rohirrim fought two battles against the Dunlendings and the Orcs of Saruman in the gap, during the first of which Théodred, son of King Théoden, was mortally wounded.