January 1 is Public Domain Day, the day that works whose copyright terms ran out in the previous year fall into the public domain. Until 1998, the law allowed all published works to fall into the public domain after 75 years. Once books, movies, and music enter the public domain, they aren't just free to copy, they're also free for anyone to appropriate, modify and build upon. Think, for example, of the many theatrical and film adaptations of Romeo and Juliet or Huckleberry Finn.

In 1998 Congress retroactively extended copyright terms by 20 years, effectively imposing a 20-year moratorium on older works falling into the public domain. As a result, works from the 1920s and 1930s that otherwise would have fallen out of copyright remain protected. We won't see another Public Domain Day release until January 1, 2019.

Keeping older works under copyright makes it harder for people to access, use, and build upon them. One fascinating study, for example, found that there are more books available on Amazon from the 1880s than the 1980s, because copyright restrictions make it hard to keep books in print. Longer copyrights make it harder to preserve older films, many of which are literally crumbling in film canisters.

It's interesting to look at the works that would be in the public domain if Congress had not changed the law in 1998. If the maximum copyright term remained 75 years, every work published in 1939 would be into the public domain starting today. Here are some of the most significant examples.

The Wizard of Oz

This was one of the first feature films to be shot in color. It wasn't a big commercial hit in its initial release, but subsequent re-releases and television broadcasts turned it into one of the best-known films from the 1930s. The movie helped launch the career of Judy Garland, who starred as Dorothy Gale.

The Grapes of Wrath

This was one of John Steinbeck's most famous novels, and it helped win him a Nobel Prize in Literature. It tells the story of the Joads, Oklahoma tenant farmers whose lives are devastated by the Great Depression and the Dust Bowl. The year's best-selling novel, it sparked a national debate about poverty.

Batman

The caped crusader made his debut in 1939. Created by Bob Kane and Bill Finger, Batman was initially a recurring character in the Detective Comics series. He would go on to have his own comics series, as well as television shows, movies, and other works related to the character. Had Congress not extended copyright terms, not only would the first Batman comics have fallen into the public domain, so would the Batman character. That means that anyone could create their own Batman comics, movies, television shows, or other works — though they'd have to be careful not to include elements of the Batman story that were introduced in later years.

Gone with the Wind

Set in the Civil War South, Gone with the Wind follows Scarlett O'Hara (Vivien Leigh) and her tempestuous relationship with Rhett Butler (Clark Gable). The film, released in 1939, is based on a book by Margaret Mitchell that was published in 1936, which would have fallen into the public domain on January 1, 2012. It was the highest-grossing film of 1939.

Madeline

This children's book spawned a lucrative media franchise, with numerous books, television shows, and movies created from the Madeline character. Written by Ludwig Bemelmans, it follows the adventures of the title character, a schoolgirl at a Catholic boarding school in Paris. As with Batman, the expiration of the first Madeline book would mean that anyone could use the character in their own works.

Finnegans Wake

This famous — and famously difficult — novel by James Joyce was first published in 1939. It took more than a decade to write and was the last novel Joyce published in his lifetime. Its unorthodox literary techniques helped make it one of the most discussed novels of the 20th century.

Anne of Ingleside

This was the final installment of L. M. Montgomery's Anne of Green Gables series, which began with the 1908 book by that name. Most of the books in the series were published before 1923 and so are already in the public domain. But the last two books to be published, Anne of Windy Poplars and Anne of Ingleside, remain under copyright protection and are not scheduled to fall into the public domain until the 2030s.

And Then There Were None

This thriller by Agatha Christie is frequently listed as one of the best-selling novels of all time. It's based on the poem "Ten Little Soldiers" (the original edition of the novel used a version of the poem employing a racial slur). In it, ten dinner guests are killed one at a time. Each dies in a manner reminiscent of one line from the poem.

"Over the Rainbow"

The same year The Wizard of Oz debuted, Judy Garland released a recording of the movie's most famous song, "Over the Rainbow." Over the last 75 years, the song has been covered by many other musicians. The Recording Industry Association of America listed it as the top song of the 20th century.