A region is a collection of connected city quads played in SimCity 4. Regions are visible in Region View, from which cities within the region can be selected for playing. New regions can be created within SimCity 4, as well as through third-party programs such as SC4Terraformer and SC4Mapper.
The default region size for SimCity 4 is 4x4 large quads (or 16x16 small quads), and is always square. However, regions of nearly any shape and size can be created using custom methods. Many custom regions of both real-world and fantasy locations can be downloaded from the various SC4 fansite exchanges.
Regions consist of a minimum of three required components. First, they must have a folder in the player's Documents/SimCity 4/Regions directory. Secondly, they must have a region.ini file, which contains the in-game name of the region, among other basic settings. Thirdly, they must have a config.bmp file, which determines the size and layout of the region. The region.ini and config.bmp files are located within the region folder. Additionally, a region may contain several .sc4 files, which are the city savegame files themselves.