It turns out the publisher Warner/Chappell Music owns the copyright to the “Happy Birthday” song. That means that every time anyone wants to use the song, he must pay a licensing fee, sometimes as high as six figures.

But how did Warner/Chappell get the rights?

“This is where it gets complicated,” says filmmaker Jennifer Nelson, laughing. —‘Happy Birthday’ Hits Sour Notes When It Comes To Song’s Free Use : NPR