As recently as the 1950 census, the District of Columbia was home to more than half ofthe roughly 1.5 million who lived in the Washington metropolitan area. And as recently as the 1970 census, it still was the most populous jurisdiction in the region, outpacing then-No. 2 Prince George's County, 756,700 to 661,700.

Now, according to the latest Census Bureau estimates reported in this paper yesterday by my colleague Stephen J. Lynton, the District has slipped to being the fourth most populous political jurisdiction in the region, which now is home to 3.4 million.

In the latest estimates for 1984, Montgomery County has edged out the District by a razor-thin 1,000 people to become No. 3. Montgomery now has an estimated 624,000 people, against the District's 623,000.

Prince George's is tops with 675,600, according to the new census estimates, followed closely by Fairfax County with 672,900, and then Montgomery and the District.

Another interesting bit of trivia we hadn't realized before, noted by a glance at the map that illustrated Lynton's story: The Washington metropolitan area not only embraces three state-level jurisdictions (the District, Maryland and Virginia) but also borders upon two other states, Pennsylvania (which adjoins Frederick County, Md.) and West Virginia (which adjoins Loudoun County, Va.). No other metropolitan area in the nation, not even New York, approaches that.