Washington, D.C., or just Washington? [closed]

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.

For tourist purposes, you would do best to use "Washington, DC", not to be confused with the many other Washingtons. The town of Washington, Virginia, less than two hours' drive from DC, has an excellent and very expensive B&B known 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 Wash?nun.

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'.

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.

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).