For the current US rules on public domain, see "Copyright Term and the Public Domain"
US Copyright Act of 1976 extended the renewal term to 47 years, making the total maximum copyright term 75 years. This was done as part of a through overhaul of copyright law in general, which changed a number of provisions. However, Works published prior to 1950 whose copyrights were not renewed had already fallen into the public domain and this act did not extend extend their copyrights.
The US Copyright Renewal Act of 1992 made renewal automatic for all works still in the first term of copyright as of its defective date. This meant that all works published in 1965 or later had their 47 year renewal term begin automatically. But it did not further extend copyright for works already in their second term, nor did it restore the copyright of works published prior to 1965 and never renewed.
The Sonny Bono Copyright Term Extension Act of 1998 extended the renewal term of works still in copyright from 47 to 67 years, for a total term of 95 years This meant that works published prior to 1937 whose copyrights had been properly renewed, did not fall into the public domain until 95 years after their publication. As of 2022, all works published in 1927 or before are in the public domain under US law, while later work are still protected if they were published with a proper copyright notice, and their copyrights were properly renewed. This law also added 20 years to the copyright terms of all works still under copyright.
In 1961, Barbara Ringer published “Study No. 31: Renewal of Copyright”, where she determined the renewal rate for fiscal year 1932 U.S. publications. In that study, she concludes that the renewal rate for Class A works for FY1932 was 7 percent. Other studies have found various renewal rates of 15% or less for works published prior to 1965.