The Commonwealth of Pennsylvania, nicknamed "The Keystone State," is a U.S. state that is located in the Northeastern and Mid-Atlantic regions of the United States. The state borders Delaware and Maryland to the south, West Virginia to the southwest, Ohio to the west, New York and Ontario, Canada, to the north, and New Jersey to the east. The state's four most populous cities are Philadelphia, Pittsburgh, Allentown, and Erie. The state capital is Harrisburg. It entered the Union on Dec. 12, 1787 and is the 2nd state of the U.S.