The area of Massachusetts was part of the original territory of the United States. It was originally included in the Charter of New England in 1620; the Charter of Massachusetts Bay, which became the Constitution of the Massachusetts Bay Colony in 1629; and a charter that united the Massachusetts Bay and Plymouth colonies in 1691. In 1785, it ceded area to the United States that became part of the Northwest Territory in 1787. Massachusetts ratified the U.S. Constitution on February 6, 1788; it was the sixth of the original 13 states to join the Union. At the time of statehood, the area of Maine was a district in Massachusetts. In 1819, it agreed to allow its district of Maine to petition for statehood, and Maine was admitted to the Union as a separate state on March 15, 1820. There were changes to the boundary between Connecticut and Massachusetts in 1804 and between Rhode Island and Massachusetts in 1862, and a resurvey of the latter boundary in 1897, resulting in generally the same boundary as the present state. Massachusetts is one of four states that are legally described as a commonwealth.