Kentucky, officially Commonwealth of Kentucky, is one of the states comprising the United States of America. It joined the Union as 15th state on June 1, 1792. The state's area is 40,410 square miles (104,660 square kilometers) and it has an estimated population of 4,269,245 (in 2008). Rivers define Kentucky's borders except on the south, where it has a straight line border with Tennessee and on its mountainous southeastern border with Virginia. The Tug river and Big Sandy river separate it from West Virginia on the east and northeast. The northern border follows the Ohio river and separates Kentucky from the states of Ohio, Indiana, and Illinois. Where the Ohio river flows into the Mississippi, the short western border with Missouri is formed by the Mississippi. The capital, Frankfort, lies between the two major cities, Louisville, which is on the Ohio River, and Lexington which is the seat of the University of Kentucky.