§ 01What the Public Domain Is
A work enters the public domain when its copyright expires, when it was never eligible for copyright in the first place, or when a creator deliberately releases it. In most jurisdictions, published works from the early twentieth century and earlier are now freely available; how long copyright lasts varies by country, so a work that's public domain in the United States may still be protected in Canada or the EU. The rule of thumb — "anything old is free" — has caused real problems for people who assumed wrongly.
Works produced by the US federal government are also public domain by statute, as are facts, ideas, titles and short phrases, which copyright never covered to begin with.
§ 02The Traps
The biggest trap is a new layer of rights over an old work. A museum photograph of a Renaissance painting, a new typeset edition of a Victorian novel, a restored recording of a 1920s jazz session — these products may carry fresh claims even when the underlying work is free. Whether those claims hold legally is contested and varies by jurisdiction, but acting as if they don't exist can mean a takedown notice or litigation.
The second trap is confusing "old" with "public domain." Publication date, country of first publication, whether formalities like copyright notice were met — all of these affect the analysis. A work published in the United States before 1978 without a proper copyright notice may have fallen into the public domain, even if it is younger than the usual term allows.
When a work genuinely is public domain, you can reproduce, adapt, sell and remix it without permission or payment. You don't need to credit the original creator — though good practice says you should.
Filed as general information, in a precise reference voice — not legal advice.