The evidence comes in the form of a book of music from 1927 titled “The Everyday Songbook,” which was put before a federal judge in California this week. There, in its pages, is a copy of the words and melody of “Happy Birthday,” whose appearance should serve to put the famous song in the public domain once and for all – and end a licensing gig that reportedly nets publisher Warner/Chappell’s $2 million per year.

In case you’re wondering how we even got here in the first place, the court case began in 2013 when a film-maker objected to paying $1,500 to license “Happy Birthday” and filed a class action suit against Warner/Chapell. She argued the song has been in the public domain for decades and no one should have to pay anything to use it.

To understand the legal issues, you have to go back to the 1890’s when sisters Mildred J. Hill and Patty Smith Hill composed “Good Morning to All,” which contains the familiar melody we know today. Soon, versions of that song began to appear with the Happy Birthday lyrics and, in 1935, a publisher called the Clayton F. Summy company obtained a copyright.

The copyright passed down through successor companies and Warner/Chappell obtained the rights as part of a $25 million acquisition in 1988. The problem, however, is that the copyright appears to have been no good in the first place due to a problem with the registration; until this week, the case hinged on whether the sisters had actually abandoned the copyright through a technicality (in those days, you had to stamp the famous (c) on a work to preserve your rights).

That has been the focus of the court case so far. But now the discovery of the 1927 songbook appears to blow away that issue since the appearance of a published version of “Happy Birthday” means the 1935 copyright is invalid. Here’s the smoking gun (courtesy TechDirt) which shows the lyrics, melody and also a “special permission” notice that appears instead of the (c) notice that would be required to preserve the copyright:

The film-maker has just submitted the 1927 songbook ahead of a hearing, scheduled for Wednesday, at which a judge is to consider the abandonment issue. Now, the judge may simply use the new evidence to rule Happy Birthday is clearly in the public domain.

Such a ruling would certainly be good news for birthday celebrants everywhere, but may also raise the larger question of why songs from the 1920’s and 1930’s are even subject to copyright in the first place. While copyright law is nominally about incentives, the U.S. entertainment industry has repeatedly lobbied to extend its terms, which critics say results in retroactive windfalls that amount to corporate welfare.