American flag waving may be a centuries-old tradition, but flag collecting - especially of those with unusual configurations or interesting variations - started hitting its stride in the last two decades.

And then came 9/11.

With patriotism at an all-time high, antique flags became a symbol of national unity. People remembered old flags stashed in their attics, bringing them into the light and onto the market.

That's around the time York antiques dealer Jeff R. Bridgman saw a big change, too, and he responded by doing more shows and increasing his advertising. The response was overwhelming.

"I was in flags before that, but when I started to push it, things really started to change," he said.

These days, historic flags and patriotic decorative arts are the focus of Bridgman's business. Not just on the Fourth of July, but year-round, he continues to see a significant increase in people wanting to collect flags. Banners from the Civil War - Union and Confederate - are in especially high demand as the country marks the 150th anniversary of that conflict.

Still, the beginning of the last decade was when historic flags' popularity took off, as important examples began to appear at shows and auctions. In 2002, the flag collection of Boleslaw and Marie-Louise D'Otrange Mastai sold at Sotheby's New York. (The Stars and Stripes: The American Flag as Art and as History From the Birth of the Republic to the Present, which records the collection, has become a major reference for collectors.)

And then Sotheby's struck gold in 2006 when it was offered the opportunity to sell four Revolutionary War flags - the last in private hands - taken as war trophies by the notorious British commander Lt. Col. Banastre Tarleton in 1779 and 1780.

One of the four, the flag of the Second Continental Light Dragoons of Connecticut, the first to feature 13 red and white stripes, brought $12.34 million, still a record not only for flags but for textiles, military, and Revolutionary War material. The other three sold as a group for a little more than $5 million.

What determines a flag's value? Experience certainly trumps appearance. That's why a tattered guidon - a swallow-tailed flag carried by cavalry troops - from the Battle of the Little Bighorn sold at Sotheby's in December for $2.2 million. The only flag not captured by the victors, the example was discovered under the body of a trooper by the burial party.

In addition to documentation, other qualities can raise or lower flag value. Bridgman lists visual impact, star configuration, age, and specific history. Condition - often the most important factor in pricing antiques - is much less important.

And Bridgman notes that size does matter: Many sewn flags in the 19th century were more than eight feet long, perfect for a public building but difficult to display in a home.

Collectors also may be interested in certain flags because they focus their acquisitions on a type - battle flags, parade flags, or flags with unusual variations in the field of stars or number of stripes. Of Bridgman's extensive inventory, his favorite is a flag of the 64th New York Volunteers. It's priced at $275,000.

"That is a 34-star flag, a presentation battle flag made for a Civil War unit that mustered in Elmira, N.Y.," he said. "The stars are configured in a star shape and there are letters intermingled that correspond to the unit's designation."

Bridgman also has the 1862 headquarters flag from Civil War Gen. Philip Henry Sheridan, priced at $550,000, with crossed swords, a star, and "No. 2" at the center, for the Second Michigan Cavalry.

Other collectors focus on political campaign flags, which were pre-20th-century banners that politicians used to advertise their candidacy. Bridgman has in stock the McKinley-Roosevelt and Bryan-Stevenson flags from their 1900 campaigns.

Interest by the general public in historic flag designs started during the Centennial celebrations of 1876, and the display of flags became a popular symbol of patriotism during the 20th century's World Wars.

Enthusiasm for historic examples began to grow in the 1990s. At that time, New Jersey collector J. Richard Pierce assembled a grouping of American parade flags, which were displayed at the 2007 Delaware Antiques Show and documented in the volume The Stars and Stripes: Fabric of the American Spirit.

In the book's introduction, Pierce wrote: "The Stars and Stripes rallies Americans in times of celebration as well as giving us the strength we need to carry on during times of peril. The flag is the symbol of our very foundation; it is the fabric that binds us together . . . it is the fabric of the American spirit."

Sheridan's headquarters flag from his time with the Second Michigan Cavalry (top) and a flag from the Battle of Little Big Horn.

The presentation battle flag of the 64th New York State Militia (top) and a parade flag from the 1844 presidential campaign of Whig Party candidate Henry Clay.