Under section 102 of the Act, copyright protection extends to "original works of authorship fixed in any tangible medium of expression, now known or later developed, from which they can be perceived, reproduced, or otherwise communicated, either directly or with the aid of a machine or device." The Act defines "works of authorship" as any of the following:
literary works,
musical works, including any accompanying words,
dramatic works, including any accompanying music,
pantomimes and choreographic works,
pictorial, graphic, and sculptural works,
motion pictures and other audiovisual works, and
sound recordings.
An eighth category, architectural works, was added in 1990.
The wording of section 102 is significant mainly because it effectuated a major change in the mode of United States copyright protection. Under the last major statutory revision to U.S. copyright law, the Copyright Act of 1909, federal statutory copyright protection attached to original works only when those works were 1) published and 2) had a notice of copyright affixed. State copyright law governed protection for unpublished works before the adoption of the 1976 Act, but published works, whether containing a notice of copyright or not, were governed exclusively by federal law. If no notice of copyright was affixed to a work and the work was, in fact, "published" in a legal sense, the 1909 Act provided no copyright protection and the work became part of the public domain. Under the 1976 Act, however, section 102 says that copyright protection extends to original works that are fixed in a tangible medium of expression. Thus, the 1976 Act broadened the scope of federal statutory copyright protection from "published" works to works that are "fixed".
Section 102(b) excludes several categories from copyright protection, partly codifying Baker v. Selden. It requires that "in no case does copyright protection for an original work of authorship extend to any idea, procedure, process, system, method of operation, concept, principle, or discovery, regardless of the form in which it is described, explained, illustrated, or embodied in such work."

Question: What decided if a published work was part of the public domain prior to 1976?
Answer: whether or not a copyright notice was affixed to that work
Alternative answer: whether the work was "published" in a legal sense