Deep inside the dark enclosure of an unopened cabinet in a library at the University of Louisville in Kentucky sat a more than 100-year-old songbook that contains the manuscript to the original song that inspired what is undoubtedly the most popular tune in the world today.

From lavish kiddie parties in the States to traditional birthday traditions in Japan and everywhere else, "Happy Birthday" is a song that is sung to celebrate another year added to a person's life. But who would have thought that the world-famous song familiar to every person in the world actually began with a different song whose manuscript stayed hidden for decades in a library in Kentucky?

James Procell, library director of the Dwight Anderson Memorial Music Library, discovered the 122-year-old manuscript of the song "Good Morning To All," which eventually became "Happy Birthday To You," in a songbook by Mildred and Patty Hill, sisters who were natives of Louisville. Mildred, who was a kindergarten teacher, composed the melody while Patty was responsible for the words of the original song.

"Obviously, it was very exciting to open up a folder full of materials that were sort of hidden away in the filing cabinet that no one ever looks at and find the manuscript for the 'Good Morning to You Song' which essentially evolved into the 'Happy Birthday' song that is so known worldwide," says [video] Procell.

The song collection, titled "Songs for the Kindergarten," was donated to the library by Hattie Bishop Speed, a local philanthropist and a friend of the Hill sisters, sometime in the 1950s along with several other documents in Speed's possession at the time. Unfortunately, the library failed to catalog the documents upon receipt, and they were relegated to the archives for several years.

Aside from the different lyrics, "Good Morning To All" has a slightly different key, and the melody of the song varies slightly, leading Procell to question if the song is the original version or if it's a revision made after the Hill sisters discovered the published version did not please their ears. Nonetheless, the song's melody remains familiar to everyone who has ever heard or sung "Happy Birthday," which is practically every person in the world.

The discovery of the manuscript comes amid a high-profile class-action lawsuit filed against Warner Music Group, which licenses "Happy Birthday" and collects up to $2 million every year to allow third parties to use the song. However, the manuscript is not likely to impact the direction of the case, as Procell says the songbook has a missing first page and no copyright statement.

Meanwhile, the Mildred Hill papers will become a significant central piece of the University of Louisville's music collection, and Procell says he plans to digitize the songbook in the coming months for preservation. He is also collaborating with the university's School of Music to organize a 2016 concert featuring Hill's compositions in commemoration of her death 100 years ago.

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