All pieces of the collection are indexed on this site and a search will retrieve a catalog description of the pieces. An image of the cover and each page of music will also be retrieved if the music was published before 1924 and is in the public domain. ALL in public domain. Reference librarians can obtain copies of the music through interlibrary loan, usually for a small fee. Ceolas houses the largest online collection of information on Celtic music, and has links to hundreds of related sites.
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Also mentioned above as a research site, this database has been responsibly annotated and most music identified as public domain is. But you should still take your PD knowledge with you anywhere you visit. From the American Variety Stage around 1920 are on line at the Library of Congress. You'll need plenty of RAM and a RealAudio player. From slapstick vaudeville routines and ethnic dialect skits to romantic ballads and dramatic recitations, sound recordings brought variety entertainment into the homes of millions of Americans.