Throughout the 20th century, changes in copyright law led to longer periods of protection for works that had been created decades earlier, which altered a pattern of relatively brief copyright protection that dates back to the founding of the nation. This came from two separate impetuses. First, the United States had long stood alone in defining copyright as a fixed period of time instead of using an author’s life plus a certain number of years following it, which most of the world had agreed to in 1886. Second, the ever-increasing value of intellectual property could be exploited with a longer term.

But extending American copyright law and bringing it into international harmony meant applying “patches” retroactively to work already created and published. And that led, in turn, to lengthy delays in copyright expiring on works that now date back almost a century.

Only so much that’s created has room to persist in memory, culture, and scholarship. Some works may have been forgotten because they were simply terrible or perishable. But it’s also the case that a lack of access to these works in digital form limits whether they get considered at all. In recent years, Google, libraries, the Internet Archive, and other institutions have posted millions of works in the public domain from 1922 and earlier. With lightning-fast ease, their entire contents are now as contemporary as news articles, and may show up intermingled in search results. More recent work, however, remains locked up. The distant past is more accessible than 10 or 50 years ago.

The details of copyright law get complicated fast, but they date back to the original grant in the Constitution that gives Congress the right to bestow exclusive rights to a creator for “limited times.” In the first copyright act in 1790, that was 14 years, with the option to apply for an automatically granted 14-year renewal. By 1909, both terms had grown to 28 years. In 1976, the law was radically changed to harmonize with the Berne Convention, an international agreement originally signed in 1886. This switched expiration to an author’s life plus 50 years. In 1998, an act named for Sonny Bono, recently deceased and a defender of Hollywood’s expansive rights, bumped that to 70 years.

The Sonny Bono Act was widely seen as a way to keep Disney’s Steamboat Willie from slipping into the public domain, which would allow that first appearance of Mickey Mouse in 1928 from being freely copied and distributed. By tweaking the law, Mickey got another 20-year reprieve. When that expires, Steamboat Willie can be given away, sold, remixed, turned pornographic, or anything else. (Mickey himself doesn’t lose protection as such, but his graphical appearance, his dialog, and any specific behavior in Steamboat Willie—his character traits—become likewise freely available. This was decided in a case involving Sherlock Holmes in 2014.)