Medieval Muslim geographers spent a great amount of time debating the exact boundaries between Hejaz and Najd in particular, but generally set the western boundaries of Najd to be wherever the western mountain ranges and lava beds began to slope eastwards, and set the eastern boundaries of Najd at the narrow strip of red sand dunes known as the Ad-Dahna Desert, some east of modern-day Riyadh. The southern border of Najd has always been set at the large sea of sand dunes known today as Rub' al Khali (the Empty Quarter), while the southwestern boundaries are marked by the valleys of Wadi Ranyah, Wadi Bisha, and Wadi Tathlith.
medieval muslim geographers spent a great amount of time debating the exact boundaries between hejaz and najd in particular but generally set the western boundaries of najd to be wherever the western mountain ranges and lava beds began to slope eastwards and set the eastern boundaries of najd at the narrow strip of red sand dunes known as the ad-dahna desert some east of modern-day riyadh the southern border of najd has always been set at the large sea of sand dunes known today as rub' al khali (the empty quarter) while the southwestern boundaries are marked by the valleys of wadi ranyah wadi bisha and wadi tathlith