He chose a 10 mile (16 km) square area that included the
existing villages of Georgetown and Alexandria, and another
called Hamburg in the Foggy Bottom area. A new city,
eventually named Washington City, was laid out in
undeveloped area within the district. The remainder of the
territory was designated Washington County (on the Maryland
side of the Potomac) and Alexandria County (on the Virginia
side).The land from the State of Virginia was eventually returned to the state in 1846. This land in Virginia makes up the modern area of Arlington County and the old part of Alexandria, Virginia, both which are considered suburbs of Washington. In fact, the Arlington National Cemetery and the Pentagon complex are both located in Arlington but are largely tied to the federal government in Washington.
The Georgetown neighborhood was originally part of Maryland and was the only significant population in the area that would become the District of Columbia. Georgetown became part of the District in 1790 when the Federal City was first created, but Georgetown remained an independent city, until 1871, when it was merged with Washington City and Washington County, completing the process of Washington and the District of Columbia occupying the same geographic borders.
The monumental core of the city consists of the National Mall and many key federal buildings, monuments, and museums, including the Washington Monument, Lincoln Memorial, and the National Air and Space Museum. Its layout is based on that proposed by the McMillan Commission report in 1901.