With very few exceptions, towns and cities in US are generally referred to with name followed by name of the state.
Reason being that there are huge amount of cities with same names.
For example besides Washington, DC, there is also: Washington, Arkansas Washington, Yolo County, California Washington, Nevada County, California Washington, Connecticut Washington, Georgia Washington, Illinois Washington, Indiana Washington, Iowa Washington, Kansas Washington, Kentucky Washington, Louisiana Washington, Maine Washington, Massachusetts Washington, Michigan Washington, Mississippi Washington, Missouri Washington, Nebraska Washington, New Hampshire Washington, New Jersey Washington, New York Washington, North Carolina Washington, Oklahoma Washington, Pennsylvania Washington, Utah Washington, Vermont Washington, Virginia Washington, West Virginia Washington, Wisconsin You might think that Washington, DC ought to be an exception, just like all other well known cities.
You don't usually have to say for example "San Francisco, CA".
But to make things more complicated, there is state of Washington, thus when you saying just "Washington", it's assumed that you're talking about the state, not the city.
BTW, many Americans use similar rule when talking about cities outside of US.
For example in casual context they'd say "Paris, France", while European would just say "Paris", France being implied.
travel.stackexchange.com/users/727/vartec
For tourist purposes, you would do best to use "Washington, DC",not to be confused with the many other Washingtons.
The town ofWashington, Virginia, less than two hours' drive from DC, has an excellent and very expensive B&Bknown as the Inn at Little Washington.
Natives will refer to "the District", or "DC", but these terms will be useless if booking a plane ticket from somewhere else.
If you want to blend in once there, the local pronunciation is closest to Washənun.
travel.stackexchange.com/users/18431/Andrew Lazarus
While formally, it's the District of Colombia, inevitably people say Washington, D.C., from my experience.
This is for two reasons.
Firstly, to distinguish it from other Washington cities in the US (eg Washington, Arkansas), and secondly, to distinguish it from Washington state - which is on the west coast.
I've also seen and heard it colloquially referred to as simply 'DC', and Wikipedia suggests some people call it 'the District'.
travel.stackexchange.com/users/101/Mark Mayo
Neither.
Legally , since 1871, it's just the District of Columbia .
About.com : This federal district was first called the City of Washington (in honor of George Washington) and the city around it was called the Territory of Columbia (in honor of Christopher Columbus).
An act of Congress in 1871 effectively merged the City and the Territory into a single entity called the District of Columbia.
Since that time the nation’s capital has been referred to as Washington, DC, the District of Columbia, Washington, the District, and DC.
travel.stackexchange.com/users/1893/lambshaanxy
You have asked for the 'formal name' of Washington, DC... As shown in the diploma below, it's "Washington, District of Columbia" and "Washington, in the District of Columbia".
While the university's existence is derived from an act of Congress, the location was "the Columbian College in the District of Columbia" (no city).
The contemporary use of "Washington, District of Columbia" reflects the formal tone and language of the document, and hence is likely to be the 'most formal' (as opposed to legal or common name).
travel.stackexchange.com/users/1737/Gayot Fow
What is the (formal) name of the capital of the United States? Is it "Washington" or "Washington DC"?   I'm aware that "DC" refers to "District of Columbia", which is the non-state internal administrative division in which Washington is located.
Washington, D.C., or just Washington?
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<usa><countries><washington-dc><north-america>
travel.stackexchange.com/questions/39654
travel.stackexchange.com/users/6811/Mark Micallef
Legally , since 1871, it's just the  District of Columbia, but people almost always say people say Washington, D.C. to distinguish it from other Washingtons. Most people refer to the capital of the US as Washington DC, however, it official name has been District of Columbia since 1871.
Cities in the US are often referred with their name and the state they are in , hence Washington DC Washington DC has been officially called District of Columbia, since 1871.