The green belt is a ring of protected open countryside around a city in which new building is largely forbidden, in order to stop the urban area sprawling outward and merging with its neighbours. The concept grew from Ebenezer Howard's 1898 garden-city proposal of a permanent agricultural belt, and was given statutory force around London by the Green Belt (London and Home Counties) Act of 1938, then extended nationally after 1947. It fixes a hard edge to the built-up area rather than letting development spread continuously.
The green belt became the primary tool of urban containment and growth management used around the world. It has preserved open land and rural character but is also blamed for pushing development further out and raising housing costs inside the ring.