Our nation’s capital is known as “Washington, DC.” But that name is kind of redundant, right? After all, “Washington” is the only thing in “DC.” So you can refer to “DC,” or you can refer to “Washington,” but they’re the same thing, at least as long as you make clear you’re not referring to Washington state. Why the redundancy? Why not just refer to the District of Columbia, period?
Here’s the history, at least as best I can tell from googling around a bit. Article I of the Constitution allowed Congress to create “such District (not exceeding ten Miles square) as may, by Cession of particular States, and the Acceptance of Congress, [to] become the Seat of the Government of the United States.” In 1790, Congress passed the Residence Act, giving the basic location of the new District but letting President Washington figure out the details. The new area was referred to as the “District of Columbia” or “the territory of Columbia.” The name “Columbia” was used because at the time it was a popular name for the colonies of the New World (which also explains the name of Columbia University, formerly King’s College).
So that’s the District of Columbia part. But why is “Washington” listed as a separate part of the District of Columbia? Under the District of Columbia Organic Act of 1801, the original 10-mile square District of Columbia consisted of five political entities. On the Maryland side, there was the preexisting town of Georgetown (now the neighborhood of Georgetown); then there was the new City of Washington (today, downtown DC); and then the County of Washington generally (consisting of the whole of the land ceded from Maryland, most of which was rural and outside the town of Georgetown and the new planned city of Washington). On the Virginia side, there was the City of Alexandria (today, most of old-town Alexandria) and the County of Alexandria (consisting of the land ceded from Virginia, most of which today is Arlington). So after the 1801 Act, a designation like “Washington, District of Columbia” would make sense: The District of Columbia had several different cities and counties within it, and the City of Washington was only one of them.
That changed over time. First, in 1846, Congress gave back the Virginia parts of the District of Columbia to Virginia, leaving only Georgetown, the city of Washington, and the County of Washington. Then, in 1871, Congress passed another Act merging the remaining parts of the DIstrict of Columbia into a single political entity. The combined entity was formally known as the District of Columbia. But as best I can tell, the city was generally still known as the city of Washington, so the whole of it became known somewhat redundantly as “Washington, DC.”
Either that, or something was needed to fill in the blanks on postal forms that required listing the city and state.