North Dakota and South Dakota are admitted as the 39th and 40th U.S. states.

North Dakota is a U.S. state in the Upper Midwest, named after the indigenous Dakota and Sioux peoples. It is bordered by the Canadian provinces of Saskatchewan and Manitoba to the north and by the U.S. states of Minnesota to the east, South Dakota to the south, and Montana to the west. North Dakota is part of the Great Plains region, characterized by broad prairies, steppe, temperate savanna, badlands, and farmland. North Dakota is the 19th-largest state by area, but with a population of just under 800,000, the fourth-least populous and fourth-least densely populated. The state capital is Bismarck and the most populous city is Fargo, which accounts for nearly a fifth of the state's population; both cities are among the fastest-growing in the U.S., although half of North Dakotans live in rural areas.

South Dakota is a double landlocked state in the North Central region of the United States. It is also part of the Great Plains. South Dakota is named after the Dakota Sioux tribe, which comprises a large portion of the population—with nine reservations in the state—and has historically dominated the territory. South Dakota is the 17th-largest by area, the fifth-least populous, and the fifth-least densely populated of the 50 United States. Pierre is the state capital, and Sioux Falls, with a population of about 213,900, is South Dakota's most populous city. The state is bisected by the Missouri River, dividing it into two geographically and socially distinct halves known as "East River" and "West River". South Dakota is bordered by North Dakota to the north, Minnesota to the east, Iowa to the southeast, Nebraska to the south, Wyoming to the west, and Montana to the northwest.

In the United States, a state is a constituent political entity, of which there are 50. Bound together in a political union, each state holds governmental jurisdiction over a separate and defined geographic territory where it shares its sovereignty with the federal government. Due to this shared sovereignty, Americans are citizens both of the federal republic and of the state in which they reside. State citizenship and residency are flexible, and no government approval is required to move between states, except for persons restricted by certain types of court orders.