The physiographic regions of the world are a means of defining the Earth's landforms into distinct regions, based upon the classic three-tiered approach by Nevin Fenneman]in 1916, that further defines landforms into: 1. physiographic divisions; 2. physiographic provinces; and 3. physiographic sections. This foundational model, which Fenneman used to classify the United States, was the basis for similar classifications of other continents later, and is still considered basically valid.