The U.S. Congress decided the locations of boundaries for the various Western territories - more or less in consultation with the administration. Because the territories were carved from maps. sometimes with little "on-the-ground" knowledge of what was contained, they often appeared as straight lines on a map.

The California-Nevada boundary followed the Sierra Nevada. The Idaho-Montana boundary is exceptional for not being a straight line - following instead natural features. The southeast boundary of Nevada follows the Colorado river.

An example of how this process could occur is provided by the Utah Territory, which originally extended from present-day Utah to the California border. In 1859 when gold and silver was discovered in western Utah Territory (the Comstock Lode and the "Big Bonanza"), the federal government was not pleased with the secessionist tendencies of Utah and there was concern that Mormon opposition to mining might inhibit the development of the important mineral strike (leadership of the LDS Church was concerned that gold rushes could bring in too many non-believers, destroying the theocracy they hoped to establish). Congress consequently created the Territory of Nevada in 1861 - but it was much smaller than the present-day state: Las Vegas was in an extended New Mexico Territory, and the eastern border was far to the west of its present location. With mineral strikes just to the east of this border, Congress cut another swath of land (one degree longitude) from the Utah Territory in 1862, and then it did this again in 1866 (another degree of longitude), establishing the straight-line Nevada-Utah boundary known today.

With the creation of the Arizona Territory out of western New Mexico Territory (again, drawing straight lines on the map) the springs that would be known as Las Vegas found itself within the new Arizona Territory, giving that territory an odd western "ear". Congress decided that it would make more sense to drop the straight line of the Utah-Nevada boundary down south to the Colorado River, and then to allow that river to become the boundary separating Nevada from Arizona, and downstream, California from Arizona. Very few people were affected by the change - the 1860 census did not even count people west of the Colorado.

Much of how these boundaries were created involved people sitting at a table in D.C. with a map, drawing lines in rather arbitrary ways, but with sparse population at the times of initial designation, it was a system that seemed to work.