"Copyleft is a general method for making a program or other work free, and requiring all modified and extended versions of the program to be free as well. The simplest way to make a program free software is to put it in the public domain, uncopyrighted. This allows people to share the program and their improvements, if they are so minded.

It used to be that you could safely assume a work was public domain unless there was a highly visible warning printed on it, containing both the copyright owner and the date of copyright (at least in the USA). This system also ensured that, when the work’s copyright expired, you could tell from any copy that this was so—by simply adding the duration of copyright to the date printed in the work’s copyright notice. The Berne Convention, however, changed all that by replacing the assumption of freedom with the assumption of monopoly, and it now takes extensive research to be sure a work is public domain.