Pittsburgh has slipped again. We've now America's 56th largest city, down from 10th in 1940, and you can bet your autographed copy of the Rand McNally road atlas that we'll continue to slide even if the regional economy booms. Because Sun Belt cities don't play by our rules.

That is a fact many people miss when comparing the sizes of cities. Only passing mention was made in a story last week of a big reason that western and southern cities grow quickly: They annex their suburbs, turning commuters into city residents, whether they like it or not.

I asked graphic artist James Hilston to show what Pittsburgh might look like had it played the same Sun Belt game for the past half-century or so. This is just a parlor trick, a little mind bender for a Sunday afternoon, not a suggestion for our future. Nobody needs to go to the parapets at the borough borders to keep Pittsburgh at bay. But here's what's been happening in five of the 10 largest cities in the United States.

Houston went from 17 square miles in 1910 to 160 square miles at mid-century to 579 square miles at century's end. San Diego has more than tripled its land area since mid-century, as has Dallas. San Antonio is nearly six times its 1950 land area. Phoenix is more than 27 times its mid-century form. In fact, the land gained by both Phoenix and San Antonio in the 1990s alone represents an area larger than Pittsburgh.

What's funny, though, is that Pittsburgh is still more densely populated than any of them. Pittsburgh has more people per square mile, even after losing half its population, than five of the top 10 cities in the country.

The chart uses data from the 2000 census. Pittsburgh has managed to misplace another 12,000 people since then, and the other cities have grown some, but the point remains. The average Pittsburgh resident has twice as many neighbors per square mile as the average resident of San Antonio or Phoenix, and at least 50 percent more than Dallas, Houston and San Diego.

The average Top 10 city is 340 square miles, roughly six times the size of Pittsburgh. They've adapted to the auto age by spreading out and broadening the tax base to include those who lived in the "real" Houston, or San Diego or Phoenix, and not just those within the buggy-age boundaries.

How much land is that?

The smallest of the cities, San Diego, would swallow about 90 of Allegheny County's 130 municipalities entirely, and take pieces of more than a dozen others. The northern line would split Hampton and McCandless, the southern line would go deep into Upper St. Clair and Bethel Park, the eastern boundary would cut through Penn Hills and Monroeville and the western one would slice Moon.

More than 900,000 people live within that circle, by my guess.

The largest city in the Sun Belt quintet, Houston, would stretch wide enough to touch all but about dozen communities in Allegheny County, and would get into Peters in Washington County. About 1.2 million people live within that circle.

These are approximations. There's no need to be precise because cities don't grow in perfect circles anyway. The population of any of the circles, however, would make Pittsburgh the ninth largest city in the country, and only slightly less densely populated than the Sun Belt dynamos.

These cities are closer in size and texture to our county than our city, you see. Pittsburghers are in no way unique by spreading out for the past several generations. Families have shrunk in size since 1940 while seeking larger houses with bigger yards here, there and everywhere.

What do you do with this information? File it. Toss it. Doesn't matter. You can rest assured Pennsylvania won't do anything with it. According to a U.S. Census Bureau survey, in the 1980s, more than 75,000 annexations occurred in the United States, involving about 9,200 square miles and a population of almost 2.6 million. During those same 10 years, 18 annexations occurred in Pennsylvania, involving less than one square mile and fewer than 500 persons.

Pennsylvania cities will empty before that changes. Some might call this a divide-and-be-conquered strategy, but Pittsburgh has shrunk too much to make annexation a plausible option anyway. Someday, the county will have to assume the role the city is no longer equipped to play, and then the perfectly American spreading of the populace won't matter so much.