In 1976, the Copyright Act was completely overhauled. The system of protecting copyrights changed giving longer protection and making it easier to claim a copyright. But pretty much all of the pre-1976 works were going to be treated differently. So the Copyright Act grandfathered in and extended the protection for these pre-1976 works. Originally, works created in 1923 or before were still in the public domain. On January 1, 2021, any works created from 1925 went into the public domain.