In the course of discussion it emerged that some people have a mistaken understanding of the meaning of "public domain".
When something is in the public domain, it means that it is not, or is no longer, subject to copyright.
One participant in the discussion said that that definition only applied in the USA, and that in the UK "it merely means that something is known to the public or a relevantly significant section of them".
That is a potentially dangerous misunderstanding. A best-selling novel or a currently popular song may be "known to the public or a relevantly significant section of them", but that does NOT mean that it is in the public domain.
Top 10 Copyright Myths page from UK Copyright Service.which says, among other things, that
Everything on the Internet ‘public domain’ and free to use[sic]Putting something on the Internet is not putting it in the public domain.
This highlights a common misunderstanding about what is meant by ‘public domain’ when referring to copyright work.
A work will fall into the public domain once copyright expires, this will typically be many years after the author’s death.
While work published on the Internet may be publicly accessible, it is certainly not in the public domain.
But, having said that, if you write your family history and publish it, whether on the Internet, in a book, or in any other form, while the wording of the text and your arrangement of the material are copyright (unless you deliberately put them in the public domain), the facts contained within the text are not copyright. If you put in the date and place of your grandfather's birth, and someone includes those facts in their genealogy, there is no breach of copyright. You can't copyright facts.
A historian who writes "The German Army invaded Poland on 1 September 1939" cannot sue another historian who writes the same thing. But if the other historian quotes several pages of the first one's book without acknowledgement, that is a breach of copyright. It is also plagiarism, which is a moral offence rather than a legal one.
If something is in the public domain, you may freely reproduce it, alter it, and sell copies of it.
How do you know if it is in the public domain?
P-10: Copyright Law | Duration Of Copyright:
What happens when copyright expires?
When the term of copyright protection has expired, the work falls into the public domain. This means that the work, has effectively become public property and may be used freely.
It should be stressed that actual duration will vary under national laws, and you should check the laws of individual countries before you attempt to use a work.
Can I claim copyright for a work that has expired?
No, once a work is in the public domain it is available to all. You cannot stop others using the work and you will have no claim to copyright on the work.
That doesn't stop unscrupulous businessmen and their lawyers from trying to claim copyright on things that are in the public domain. Can you imagine the term "tea" being copyright, or a registered trademark, so you have to write "Tea (TM)" every time you use it? One American firm tried it on with roobos tea, a herbal tea that has been used in South Africa for generations. It took a long and expensive court battle to get them to drop their pretensions and acknowledge that it was in the public domain. You can read more about that here. There are some more examples at Notes from underground: Do Americans have any concept of "justice"?.
Things can just as easily go the other way.
Solomon Linda, a South African musician, composed a song Mbube back in 1939. The Disney Corporation used it in their film The Lion King, and claimed that the song was in the public domain. It wasn't. It took a long and expensive court case to get them to pay royalties to Linda's heirs; Linda himself died in poverty, though lots of record companies and others made a lot of money out of his song. And you can read more about that here.
I doubt that anyone is going to make a fortune out of genealogy, but nevertheless, be careful, be very careful, about how you use the term "public domain".