Long hidden in the shadows of New York City and, to a lesser extent, Washington D.C., Philadelphia is one of America's most overlooked cities. It's in southeastern Pennsylvania, in the mid-Atlantic region, and is the sixth most-populous city in the United States. Often referred to as "Philly," the city is coterminous with Philadelphia County. Philadelphia sits adjacent to New Jersey and the Delaware River, and only 15 miles from Delaware, and as such, its metropolitan area encompasses counties in Pennsylvania, New Jersey and Delaware, and even Maryland.