The construction and routes of railroads, the results of the American Revolution, the terms of the 1808 proposal for the Erie Canal and controversies regarding slavery were major historical events that influenced state boundaries, according to Mark Stein, author of "How the States Got Their Shapes." Political ideologies also influenced where state lines fall.
As the British colonized the United States, it created unequal state boundaries. Thomas Jefferson considered this unfair and suggested that, in the wake of the American Revolution, the country create states equal in size. He suggested using longitude and latitude coordinates to denote each state's boundaries. For land in the Northwest Territory between the Ohio and Mississippi Rivers, he suggested states be 138 miles high and about 276 miles wide, or about two degrees of latitude and about four degrees of longitude. Congress did not apply this logic in the Northwest Territory, but used it as the basis for creating state boundaries elsewhere in the country.
Kansas, Nebraska, South Dakota and North Dakota each cover about 3 degrees of latitude, making them about 207 miles, north to south. Colorado, Wyoming and Montana each have about 4 degrees of latitude, or about 276 miles. Washington, Oregon, Colorado, Wyoming, North Dakota and South Dakota have about 7 degrees of longitude, equal to about 483 miles wide.
The United States also based state boundaries on rivers, and as railroad lines opened across the country, used tracks to determine where one state ended and the next began. After the Louisiana Purchase in 1803, leaders decided to base state boundaries in this region on where slavery was legal, and the Missouri Compromise in 1818 solidified this practice. The Compromise allowed slavery in states with northern borders below 36 degrees, 30 minutes North latitude, excluding Missouri.
Texas and California, two of the largest states in the country, created their own borders, according to Stein. The discovery of gold in California and the fear that Texas would secede left the country little choice but to accept their boundaries as they proposed them.