For nearly a century, a large oval-shape linen tent where George Washington is believed to have slept during the Revolutionary War sat on display in Valley Forge, Pa., with a gaping hole in its roof.

But now a combination of luck and forensic detective work has led to the discovery of the missing section of fabric -- snipped out, historians believe, by a memorabilia seeker -- and to the discovery that the tent was originally striped blue and white.

''It is the missing piece,'' said Loreen Finkelstein, a textile conservator who made both discoveries while restoring the tent for the American Revolution Center, a nonprofit organization collecting artifacts and raising money for a Revolutionary War museum.

The tent, 25-feet-10-inches long by 17-feet-7-inches wide by 13 1/2-feet high, is a faded beige, but Mrs. Finkelstein has learned that it was originally striped blue and white and had red wool trim.

Historic documents describe another sleeping tent with red and white stripes that was bought in May 1776 as part of a set of tents for Washington. Mrs. Finkelstein's discovery appears to confirm for the first time that there was more than one set. Considering the wear and tear of traveling from one encampment to another, it is not surprising that Washington's quartermasters may have had several sets of tents.

''This has been recognized as part of the commander in chief's equipment since the 18th century,'' Dr. R. Scott Stephenson, who was hired to assess the American Revolution Center's artifacts, said of the tent that has been on display.

The American Revolution Center plans to make the tent a featured exhibit in its museum in Valley Forge. When the group acquired it in 2002 from the Valley Forge Historical Society, it needed to be completely restored.

Mrs. Finkelstein was hired to prepare the tent for exhibition. With its poles stored and its tired folds resting across her home workshop, Mrs. Finkelstein began laying sheets of Mylar, a polyester product 10 to 20 times the thickness of kitchen plastic wrap, over the tent. In this way she created a Mylar impression of its entirety -- every stain, every hole, every repair, even every thread.

As she was working, she discovered the blue and white stripes. ''I noticed,'' she said, ''that there was a very faint, very occasional, what looked like a line on the fabric.''

Mrs. Finkelstein called a former Federal Bureau of Investigation agent, Douglas Deedrick, for help. She sent him tiny samples of blue and white threads from the tent. Mr. Deedrick studied them under a variety of microscopes and lights to measure how much light they absorbed and reflected, and at what wavelengths they did it.

But he saw the stripes best with his naked eye and Mrs. Finkelstein's camera.

In the fall of 2004, while Mrs. Finkelstein was vacuuming and washing the tent, she was also busy with something else: the George Washington sculpture project at Mount Vernon, his estate in Virginia.

One day she and Carol Borchert Cadou, a senior curator at Mount Vernon, were studying one of Washington's waistcoats. As she worked with gloved hands, Mrs. Finkelstein casually talked about the hole in the tent.

''I said,'' Ms. Cadou recalled in an interview, '' 'We have several fragments in our collection that are associated with Washington's tents.' ''

By chance, an art handler overheard them and mentioned that some of the fragments were kept in the very room where the women stood.

As Mrs. Finkelstein looked at one of the fragments, on loan from Yale University, she said, ''This looks like the missing piece.''

She did a thread count, collected a few fibers for forensic tests and traced the fragment's outline on a Mylar sheet. But she told no one else what she suspected.

''Why cause a problem if you don't know for sure?'' she said in an interview.

When Mrs. Finkelstein returned home and set her Mylar template into the ceiling's hole, it fit like a missing puzzle piece.

More testing showed that the piece of fabric and the tent had the same weave, the same thread count and the same decorative blue threads. Most significant, they shared a seam, and the top and bottom stitching techniques along that seam also matched.

Through all this analysis, however, the tent and the fragment remained separated. The American Revolution Center, Yale and Mount Vernon finally brought them together, and on Jan. 20, when Mrs. Finkelstein set the fragment in the ceiling's hole, it was an exact match.

She believes there is still another small fragment missing, but as she wrote in one of her formal reports, ''To have found the missing fragment of this extremely important textile fabric is extraordinary.''

Photos: RESTORING HISTORY -- Although conservators worried that the edge, or selvage, of a torn bit of linen wouldn't fit the hole in the tent, shown in 1909, top, it matched perfectly. (Photos by Top, the American Revolution Center; above, Lisa Godfrey for the American Revolution Center)