There are two primary types of boundaries—geometric and natural. Geometric boundaries are straight and follow lines of latitude, longitude, or a certain compass direction between points. Geometric boundaries were established to divide territories before settlers entered areas. Most of the states in the Western United States have at least a portion of their borders formed by geometric boundaries (especially rectangular-shaped Colorado and Wyoming). Natural boundaries are usually curvy because they follow the crests of mountains or the center of rivers. Natural boundaries are very common in places like Europe, where the region was populated before the countries were created.