Public Domain

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There are two primary definitions for the term public domain. The one we won't be discussing is public land that can be purchased from the government for a small amount of money and if you homestead it for a few years, it becomes yours.

The term "public domain" that we will discuss here refers to the collective of works not covered by intellectual property rights in any way whatsoever. This is usually due to said rights either expiring or never existing, but it can also be due to the creator intentionally forfeiting said rights. Examples of such works include the English language, the formulae of Newtonian physics, recipes, the works of Shakespeare and Beethoven, and the patents on powered flight. This also applies to many old movies, cartoons, novels, poems, comics, and various other works of entertainment, science, and general knowledge. In general, Public Domain works make for a great resource for those who would like to use properties in their work, but don't have a lot of cash.

Since Public Domain works don't have the protection of copyright or other IP rights, anyone can use them as the basis for new creative works without fear of running into lawsuits or other legal issues. In essence, someone is free to use, adapt, change, or implement entire sections of the work as they so choose (including the entirety of the work), even for profit, all without asking permission or paying royalties. Shakespeare's works, for example, have received numerous film adaptations with their own unique spin on The Bard's tales. People can also use those works, in part or in whole, within a new creative work (someone can quote as much Shakespeare as they want within a film).

In general, works enter the public domain by one of several means:

The work existed before copyright laws existed (generally anything before the early 18th century)
The work got published after the creation of copyright, but it existed long enough for its original copyright to have expired (anything published before 1928 in the United States, except for sound recordings�with changes to US copyright law made in 2018, pre-1925 recordings entered the PD in 2023).
The work's copyright lapsed due to age (with the current legal standard for the US holding that age at seventy years after the death of the creator, or 120 years after creation/95 years after publication [whichever is less] for works where a corporate entity is the creator).
The copyright holder waives their copyright, or the owners aren't around to renew it and no one else has claimed ownership of the property—for example, this is what allowed the entire Van Beuren cartoon studio library to become public domain, since the independent studio abruptly went belly up in 1936, its founder died soon after, and their generally unpopular films faded into obscurity after the fact, save as quick to make home movie rentals decades later.
The work got published at any time between 1928 and 1963, but the copyright owner failed to renew their copyright. (After 1963, renewal became optional.)
The work was published between 1928 and March 1, 1989 with a faulty copyright notice. (e.g. missing a copyright symbol [or the word itself], year or claimant)
In the United States at least, if the work was originally created by the federal government, it is automatically in the Public Domain at creation. (This includes the Private Snafu cartoons and Frank Capra's Why We Fight films.) The word "federal" is important in this context�works created by state governments may or may not be PD, depending on the laws of the relevant state.
- That said, other federal laws can take certain government-created properties out of the PD. One notable example is Smokey Bear, the mascot of the US Forest Service. Created in 1944 under the direction of the Forest Service, he's been under the control of that agency's parent, the US Department of Agriculture, since 1952, when Congress passed and President Truman signed a bill to that effect.
No one knows who is the creator of the work, works that spread through Oral Tradition often go through this, like Folk Music, Playground Songs, and Urban Legends.
It was created by a machine or an animal. (Stern Electronics Inc vs. Kaufman ruled that video game images may be copyrighted, as they are the product of intentional human design.)
- Works generated by Artificial Intelligence (AI) are currently considered copyrighted by the owner of the AI^note, though artwork generated by the AI are often in the public domain.

In the United States, a vast majority of works produced prior to 1928 have fallen into the public domain (with a few exceptions existing thanks to legal issues); works produced thereafter may or may not fall under copyright, owing to a litany of circumstances (including various extensions of copyright terms).

Starting in 2019, after a 20-year dry spell due to the Sonny Bono Copyright Term Extension Act, works made in 1923 have entered the public domain. Throughout The New '20s, works from 1924 to 1933, at least those whose copyright have not been renewed, are expected to enter the public domain.

For certain works, international laws and treaties complicate matters further. While the King James Bible falls under the public domain in the US, over in the UK, the Crown holds it under perpetual copyright. A handful of (currently, African and South Americans) have a so-called "paying public domain" by which one doesn't have to ask permission for the use of public domain works, but must pay a fee in lieu of royalties to a government entity. Copyright scholar Pamela Samuelson has described the public domain as "different sizes at different times in different countries".

See also Free/Libre Open Source Software, for another form of licensing that allows profiteering and modifications to the original source.

Public Domain Tropes:

Public Domain Artifact: Various objects that originate from old folklore and ancient legends.
Public Domain Canon Welding: Canon Welding public domain works, either with a new work or with each other, to create a new work.
The Public Domain Channel: An in-universe Show Within a Show that is actually public-domain media in the real world.
Public Domain Character: Fictional characters that are owned by nobody, thus allowing anybody to use them.
Public Domain Soundtrack: Music that is completely free of any royalties, and thus can be inserted into any media.