His title will be mayor, which means the chief executive of a city, but the governmental unit over which Marion Barry will preside is unique in the nation.

Legally, the District of Columbia is described as a municipal corporation -- a city -- but it combines many of the functions of a state and a county as well. Its 1o-member City Council enacts the kinds of laws that, in most states, would be produced by their legislatures.

Yet in the end, both the laws passed by the council and the city's annual budget are subject to review by Congress, which retains ultimate legal power.

The Districkt has a voice in Congress, a nonvolting delegate, but no representatives or senators. A proposed constitutional amendment granting full representation was approved by Congress last year, and has been ratified by three of the necessary 38 states.

With a budget of $1.4 billion a year the District of Columbia is the only city that has its own state prison system automolbile license tags and state university. Conversely, the federal government runs the city's park system, its museums, its airports and provides its water supply.

Contrary to a widely held belief that Uncle Sam pays the full cost of District operations, the District collects the full range of municipal and state taxes from its 740,000 citizens -- a total of $1 billion a year. Because federal oroperty is untaxable, the U.S. Treasury contributes more than $400 million to city operations.

The District comprises 65 square miles of land carved out of Maryland in 1800 (another 35 square miles was taken from Virginia, but returned in 1846).

Washington, a city much smaller in area than the entire original District of Columbia, had a succession of mayors from 1802 to 1871. For four years it had a ternitorial governor. From 1874 to 1967 it was governed by boards composed of three presidentially appointed commissioners.

Walter E. Washington was the last of the commissioners and the first mayor of the entire District of Columbia. He was first appointed by thenpresident Lyndon B. Johnson in 1967 and won popular election in 1974. He lost last year to Barry.