Physiographic regions of the world are a means of defining Earth's landforms into distinct regions, based upon the classic three-tiered approach by Nevin M. Fenneman in 1916, that separates landforms into physiographic divisions, physiographic provinces, and physiographic sections.[1] Originally used in North America,[1] the model became the basis for similar classifications of other[clarification needed] continents, and was still considered valid as of 1951[update].[2]