When Congress authorized the American flag in 1777, it created a grand banner but a peculiar design dilemma.

The legislation said the flag must have 13 stripes of red and white, with a blue field containing a constellation of white stars - but never said how those elements should be organized.

The result, in an age before mass communication and mass production, was that seamstresses and flag-makers invented their own odd, often outlandish interpretations.

Some covered half the flag in blue. Others set the stripes vertically. Some put the stars on the stripes. Some added eagles, shields, or political slogans, because, after all, there was no rule against it - and no efficient means to compare one design to another.

Today, collecting those red, white, and blue relics - and telling their stories - is the passion of Jeffrey Kenneth Kohn, a retired Elkins Park psychiatrist and one of the nation's foremost experts on the American flag.

"This is really an American form of folk art," Kohn said, examining an unusual incarnation from his collection of nearly 1,000 flags. "People could do their own thing."

And, boy, did they.

During the nation's first 135 years, there were as many versions of the American flag as Americans could conceive.

Kohn's favorite: a 13-star flag created to honor the 1824 visit of Lafayette, hero of the Revolution, and updated during the Civil War, when its owner added 21 smaller stars to recognize Kansas' 1861 entry into the Union.

"When you look at it," Kohn said, "it doesn't look like anything you've ever seen."

Kohn, 62, has examined, appraised, bought, sold, organized, and displayed thousands of American flags. He's the flag consultant to Sotheby's Inc., the renowned auctioneer, and has produced exhibitions for Republican National Conventions in Philadelphia and Minneapolis. He's a coauthor of American Flags: Designs for a Young Nation, and regularly lectures at schools and museums.

All of which is a long way from cleaning out dusty attics and dank basements.

That was how he put himself through medical school at Thomas Jefferson University - the find of a quality antique helping to pay his tuition.

Growing up in South Philadelphia, the son of a portrait-photographer father and a bookkeeper mother, Kohn always collected things. Rocks. Stamps. Fossils.

He continued collecting even as he began his medical practice, honing an interest in early American art and military uniforms. A buddy interested in flags asked Kohn to keep an eye out at auctions. In 1982, Kohn began to seriously collect flags himself, aided by his earlier studies of uniform fabrics and sewing techniques.

"There's something about an American flag," mused Kohn, seeking to explain the fascination. It might seem repetitive, exploring the same stars and stripes. Instead, he has found, flags are as different as people.

"His knowledge is so valuable," said Lisa Moulder, director of the Betsy Ross House.

A few years ago, the house was poised to accept an Ohio collector's offer to display a rare 18th-century flag, deciding at the last minute to seek an opinion from Kohn - who quickly identified the flag as newer than advertised.

"We would have been really embarrassed," Moulder said.

Last year, Kohn lent many of his pieces to the Betsy Ross House for a major Flag Day exhibition. This year, tomorrow and Sunday, he will offer free, oral flag appraisals to anyone who donates $5 to the house.

"He's one of America's leading authorities on the American flag," said Michael Axelrod, director of development and educational programming for the Philadelphia Flag Day Association, which Kohn serves as historian. "He helps us locate, and then authenticate, the pieces that we add to our collection."

The first flags weren't created as decorative or even patriotic devices; they were a means of communication, for ships at sea and for soldiers on the ground. Over time, as stars were added when states joined the Union, people didn't see their old flags as collectible. They saw them as obsolete. And got rid of them.

That has contributed to scarcity in the market, where an exceptional flag can sell for $100,000 and a particular group of four, rare Revolutionary flags sold for $17.4 million. Those flags had been captured by a British officer in 1779-80 and put up for auction by a direct descendant.

Perhaps only one in 500 flags will carry an authoritative provenance. The mystery is part of what drives Kohn. At his house, shelves are crammed with books on advertising, railroads, art, and politics. A Victrola stands at the ready. On the walls are original works by turn-of-the-20th-century artist Harrison Fisher, whose illustrations brightened the covers of Cosmopolitan magazine. He brings out some flags:

A 12-star flag, strange since the United States never had fewer than 13 states. A flag with 53 stars, made in 1896, to recognize 45 states and eight territories. A flag imprinted "Nixon," from one of his campaigns. Another bearing a portrait of William Henry Harrison, from an age when flags were advertising vehicles. An 1889 swallowtail flag carried by the 10th Cavalry, the all-black regiment known as the Buffalo Soldiers.

All this patriotic chaos came to an end in 1912. That year, President William Howard Taft put order to the flag, issuing a directive that set its proportions and arrangement.

That eliminated many of the curiosities that make early flags so intriguing.

"They're really dynamic," Kohn said. "It gives you a window that's just amazing."