Gap of Rohan
The Gap of Rohan is the opening between the mountain ranges of the Misty Mountains and the White Mountains.
Through the Gap of Rohan flowed the rivers Isen and Adorn, which formed the effective boundary of the Kingdom of Rohan. Rohan was formerly known as Calenardhon, and thus the Gap was then the Gap of Calenardhon. The triangle between the Isen and Adorn rivers and the White Mountains was a contested area between the Rohirrim and the Dunlendings.
The ancient Great 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.
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 a number of battles against the Dunlendings and the Orcs of Saruman in the gap, during one of which Théodred, son of Théoden King, was mortally wounded.