Louisville is the largest city in the Commonwealth of Kentucky and the 28th most populous city in the United States. On the basis of population it is rated the state's only 1st-class city. Louisville is the historical seat and, since 2003, the nominal seat of Jefferson County.
Sited beside the Falls of the Ohio, the only major obstruction to river traffic between the upper Ohio River and the Gulf of Mexico, Louisville first grew as a portage site. It was the founding city of the Louisville and Nashville Railroad, which grew into a 6,000-mile (9,700 km) system across 13 states. Today the city is best known as the location of the Kentucky Derby, the first of the three annual races that make up the Triple Crown of Thoroughbred Racing. It is the home of the University of Louisville and three of Kentucky's six Fortune 500 companies. Its airport is also the site of UPS's worldwide air hub.