How do movies get lost? Surprisingly easily.

"It's hard for people to understand that movies are not a DVD, they're not one reel that you hold in your hand," said Rachel Parker, a curatorial technician with the Library of Congress and an organizer of the festival. "A feature film can be eight reels long. With nitrate deterioration you might lose that first reel, the one that gives you the film's credits and title."

Before digital took over, movies were shipped to theaters in metal cans. A projectionist would splice or assemble the reels together onto a platter and then thread it through the projector mechanism. Many films never made it back to the studio after their theatrical runs. Projectionists might collect favorite scenes from the features sitting around in their booths.

"Sometimes all that survives from those original reels is what the projectionist cut out," Parker said. "So now we'll have 500 feet of a film that used to be 2,800 feet long, but it's from the middle of something, and we just don't know what it is."

Silent-era bootleggers also would sometimes intentionally obscure a film's title on a knock-off reel, so the original owner wouldn't find out. But the most common way footage was lost was from a projector chewing it up, according to Rob Stone, the moving image curator at the Library of Congress and the originator of the workshop.

The beginnings and ends of reels are the most vulnerable part of a movie. Unfortunately, that's usually where you find the title and credits. It's as if a librarian had to identify a book without its cover or title page. Stone compares it to turning on a TV show five minutes after it's started. "What are you watching?" he asks. "How do you tell?"

Some of the problems with unidentified films stem from archivists themselves. Movie reels are separated, cans are mislabeled, and the sheer volume of material can be overwhelming. "Those pictures you see of archivists meticulously winding through every frame of film, well that really doesn't happen," Stone said.

"Our early efforts were purely with unidentified films," he continued. "But now we've moved on to misidentified or under-identified films as well. Say a movie that was retitled, and then entered into archive records under that title. Frankly, sometimes we're just dealing with inadequate cataloguing."

Occasionally, a film has been hiding in plain sight. Stone cites a case in a French archive in which cans labeled "Unknown" were set aside in a corner. They actually contained reels from a 1927 Lon Chaney feature called The Unknown. He recalls a similar story from his time at the UCLA Film and Television Archive: "There was a film called Identity Unknown. Well, the cans said 'Identity Unknown,' so they put them away, in fact in seven different locations."

This September the National Film Preservation Foundation will be releasing a DVD with two recent discoveries: Upstream, a 1927 backstage comedy directed by John Ford, and The White Shadow, the earliest surviving film credit for Alfred Hitchcock. Both films had been presumed lost until they were identified in a New Zealand archive.