Quadripoint
The Four Corners Monument is the only point in the United States where four states meet: Colorado, Utah, New Mexico, and Arizona meet at right angles. The United States first acquired the area now called Four Corners from Mexico after the Mexican–American War in 1848. In 1863 Congress created Arizona Territory from the western part of New Mexico Territory. The boundary was defined as a line running due south from the southwest corner of Colorado Territory, which had been created in 1861. By defining one boundary as starting at the corner of another Congress ensured the eventual creation of four states meeting at a point, regardless of the inevitable errors of boundary surveying. The monument is centered at .
East Coast of the United States
The fourteen states that have a shoreline on the Atlantic Ocean are, from north to south, the U.S. states of Maine, New Hampshire, Massachusetts, Rhode Island, Connecticut, New York, New Jersey, Delaware, Maryland, Virginia, North Carolina, South Carolina, Georgia, and Florida. In addition, Pennsylvania and the District of Columbia border tidal arms of the Atlantic (the Delaware River and the Potomac River, respectively). The states of Alabama, Mississippi, Louisiana, and Texas, (via the Gulf of Mexico) as well as the territories of Puerto Rico, the US Virgin Islands, and Navassa Island (the latter only bordering the Caribbean Sea) have Atlantic coastline, but are not included in the definition.