The period from 1923-1963 is of special interest for US copyrights, as works published after January 1, 1964 had their copyrights automatically renewed by the 1976 Copyright Act, and works published before 1923 have generally fallen into the public domain. Between those dates, a renewal registration was required to prevent the expiration of copyright, however determining whether a work's registration has been renewed is a challenge. Renewals received by the Copyright Office after 1977 are searchable in an online database, but renewals received between 1950 and 1977 were announced and distributed only in a semi-annual print publication. The Copyright Office does not have a machine-searchable source for this renewal information, and the only public access is through the card catalog in their DC offices.