Originally the state's western border was a straight line, defined as the meridian passing through the Kawsmouth, the point where the Kansas River enters the Missouri River. The river has moved since this designation. This line is known as the Osage Boundary. In 1836 the Platte Purchase was added to the northwest corner of the state after purchase of the land from the native tribes, making the Missouri River the border north of the Kansas River. This addition increased the land area of what was already the largest state in the Union at the time (about to Virginia's 65,000 square miles, which then included West Virginia).