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Philadelphia (/ˌfɪləˈdɛlfiə/ ⓘ FIL-ə-DEL-fee-ə), colloquially referred to as Philly, is the most populous city in the U.S. state of Pennsylvania and the sixth-most populous city in the United States.[11] Its population was 1.60 million at the 2020 census and estimated at 1.57 million in 2025.[6] The Philadelphia metropolitan area, also known as the Delaware Valley, is located in both the Mid-Atlantic and northeastern United States; home to 6.33 million residents, it is the nation's ninth-largest metropolitan area.[8] Philadelphia is known for its culture, cuisine, and history, maintaining contemporary influence in technology and business, sports, and music.[12][13]

Philadelphia was founded in 1682 by William Penn, an English Quaker and advocate of religious freedom, and served as the capital of the colonial era Province of Pennsylvania.[3]: 4–5 [14]: 21–22  It then played a vital role during the American Revolution and Revolutionary War. It served as the central meeting place for the nation's Founding Fathers in hosting the First Continental Congress (1774) and the Second Continental Congress, during which the Founders formed the Continental Army, elected George Washington as its commander, and adopted the Declaration of Independence on July 4, 1776. During the Revolutionary War's Philadelphia campaign, the city fell to the British Army, which occupied Philadelphia for nine months from September 1777 to June 1778.[15] Following the end of the Revolutionary War, the U.S. Constitution was ratified at the Philadelphia Convention. Philadelphia remained the largest U.S. city until 1790, and it served as the nation's first capital from May 10, 1775, until December 12, 1776, and on four subsequent occasions until 1800, when construction of the new national capital in Washington, D.C. was completed.[16]