Free Knowledge Culture Calendar/September 22

 Today in 2015, the last valid patent on MP3 expired (US patent 5,812,672) and the extremely popular audio format became a patent-free and open standard.

Although better formats are now available that also cover a wider range of uses, this contributes to the likely immortality of the format that began its career in the 1990s as an enabler of music file sharing in near-transparent quality.