Proof of Public Domain
You should use a public domain composition only if you have proof of public domain from a legitimate source. If you do not have a legitimate source in your possession, there is no way you can be certain that the music you use is in the public domain. A legitimate source is a tangible copy of the work with a copyright date old enough to be in the public domain. Sources are almost always either an original or a copy of a book or sheet music. You cannot just "know" a song is in the public domain or just "see" the name of the song in a book, on a list, or even on this web site. An attorney will tell you that there really is no such thing as absolute "proof of public domain". But you must protect yourself with the best "proof" you can find. If you do not do your own research and obtain a legitimate public domain copy of each work you use, you can easily make errors which could result in your having to pay substantial royalties.
Copyright Date Old Enough to Be in the Public Domain
referred to as "PD Copyright Date"on this web site
For sheet music, the copyright date is usually shown on the first page of the music notation. If you are obtaining copies of original sheet music as proof of PD, you should get a photocopy of the cover page as well as the actual pages of music notation. While getting a copy of the cover page may not be absolutely necessary, you may not be able to show whether the copy was made from sheet music or from a book of music without a copy of the cover page. It is always best to be able to absolutely identify your PD source.
For books there are two applicable copyright dates:
Source and Public Domain
It is quite possible that a book can be a legitimate source for proof of public domain, but the book itself is NOT in the public domain. A legitimate source may not be public domain itself A book with a 1930 copyright notice might include a work with a 1900 copyright notice. Dover Books currently publishes a wonderful series of books containing reprints of old music. In both of these instances, works included in the book are in the public domain, but the actual source book is still protected by copyright. The work--the music or lyrics or arrangement--are in the public domain and you may use them. You MUST clearly understand the distinction between the public domain status of a work and the public domain status of the source of the work.
Legitimate Sources for Proof of Public Domain
Questionable Sources for Proof of Public Domain
Invalid Sources for Proof of Public Domain