“Happy Birthday” (aka “Happy Birthday To You”) may be sung millions of times a day at birthday bashes around the world, but putting it in a movie or recording a version of the brief ditty will set you back some money for royalties to Warner Music Group, which has long claimed to hold the copyright for the song. But a recently filed court document claims that there is conclusive proof that the song belongs in the public domain.

The makers of Happy Birthday, a movie about the classic song, have sued Warner/Chappell Music to get back the $1,500 they had to pay to use the song in the film. They are also hoping to represent a class of others who have paid what they contend is a bogus royalty on the “Happy Birthday” song.

In a recent filing [PDF] with the federal court hearing the case, the filmmakers claim they can show “conclusively that Happy Birthday has been in the public domain since no later than 1922.”

Warner/Chappell’s copyright claim is based on a 1935 version credited to writers Preston Ware Orem and Mrs. R.R. Forman, rather than Patty and Mildred Hill, the sisters who actually wrote it decades earlier.

But the filmmakers and others have argued that this copyright is only for a particular piano arrangement of the song, and that the new evidence shows the Hills’ version of the song had already been given over to the public domain by 1922.

According to the plaintiffs, they recently received 500 pages of documents from Warner/Chappell as part of the discovery process. Included in that cluster of documents was the 15th edition of The Everyday Song Book from 1927, which they claim is the “proverbial smoking-gun.”

Included in the songbook is “Good Morning and Birthday Song,” which uses Patty Hill’s words for “Happy Birthday” and the very similar “Good Morning” with sister Mildred Hill’s music.

And while there is a line of text below this song that reads “Special permission through courtesy of The Clayton F. Summy Co.,” there is no specific copyright claimed. However, the filmmakers note that every other individual song in the book has an explicit declaration of copyright.

They were able to obtain a revised Fourth Edition of the songbook from 1922, and again the song contains the permission notice but no copyright claim. This, argue the filmmakers, “is fully consistent with Plaintiffs’ position that the Happy Birthday lyrics had been dedicated to the public many years before then.”

As Ars Technica’s Joe Mullin points out, the lack of an explicit copyright notice in the 1922 songbook is “critical, because under the 1909 Copyright Act which was then in force, a published work had to include the word ‘Copyright,’ the abbreviation ‘Copr.,’ or the ‘©’ symbol, or ‘the published work was interjected irrevocably into the public domain.'”

Additionally, even if the court holds that the “permission” line constitutes a valid copyright in 1922, the laws in place at the time would have put the song into the public domain by 1949. And, even if that copyright had been renewed, it would ultimately have expired at the end of 1997.

The filing from the filmmakers came only days before a scheduled hearing on the copyright issue, so there may be an update coming later this week.

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