A number of Mudcatters have done copyright permissions work, so we are a pretty good place to check first. We CAN give advice, but as Stilly River Sage says, we are not an official source. Here are the US guidelines for Public Domain. If you live in another country, Your Results May Vary:

As a general rule, for works created after January 1, 1978, copyright protection lasts for the life of the author plus an additional 70 years.

https://www.copyright.gov/help/faq/faq-duration.html

Wikipedia has a pretty good article on copyright: https://en.wikipedia.org/wiki/Copyright A quote:

In the United States, all books and other works, except for sound recordings, published before 1926 have expired copyrights and are in the public domain. The applicable date for sound recordings in the United States is before 1923. In addition, works published before 1964 that did not have their copyrights renewed 28 years after first publication year also are in the public domain. Hirtle points out that the great majority of these works (including 93% of the books) were not renewed after 28 years and are in the public domain. Books originally published outside the US by non-Americans are exempt from this renewal requirement, if they are still under copyright in their home country.

But if the intended exploitation of the work includes publication (or distribution of derivative work, such as a film based on a book protected by copyright) outside the US, the terms of copyright around the world must be considered. If the author has been dead more than 70 years, the work is in the public domain in most, but not all, countries.

In 1998, the length of a copyright in the United States was increased by 20 years under the Copyright Term Extension Act. This legislation was strongly promoted by corporations which had valuable copyrights which otherwise would have expired, and has been the subject of substantial criticism on this point.

It used to be pretty easy from 1998 to 2018 - if it was published before 1923, it was pretty clear you didn't have to pay royalties and license a song. Now things change every year, and we get another year of publications added to the Public Domain. It's complicated, so it's best to ask a professional if you're in doubt. But we can give you pretty good preliminary advice here.

I did the copyright clearances for the 1200 songs in the Rise Again Songbook. I gathered documented evidence about the earliest publication date of every one of those songs - and then our publisher, Hal Leonard made the final decision based on the information I submitted. It was hard work, but I enjoyed it. And basically, the rule on public domain is this: you have to satisfy your publisher. We had several songs in Rise Again that were clearly published before the 1923 cutoff date, but we still had to pay royalties and get a license because there was a copyright claim by a particularly litigious party. Almost all Carter Family songs should be in the public domain, but Ralph Peer's company is still clinging to its claim on most of those songs. Same with "Big Rock Candy Mountain" and other hobo songs recorded by Harry McClintock. And we had to pay rights for "I'm Henry the VIII," even though it was a music hall song published long before 1923 - it had never been published in the USA, so the US clock started when the Herman's Hermits recording was released in 1966.