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MP3 is used to store elementary streams conforming to MPEG-1 Part 3: Audio (ISO/IEC 11172-3:1993) and MPEG-2 Part 3: Audio (ISO/IEC 13818-3:1995). MP3 revolutionized the music industry and also a small hack has probably contributed to this development (*), i.e., a time-limited reference encoder has been made available to demonstrate the capabilities of this new standard. The time limitation got hacked and the story as we know it began...
The MP3 story also reminds me of an interview from 2004 by Leonardo Chiariglione where he stated:
When we approved the standard in 1992 no one thought about piracy. PCs were not powerful enough to decode MP3, and internet connections were few and slow. The scenario that most had in mind was that companies would use MP3 to store music in big, powerful servers and broadcast it.
Nowadays, MP3 players can be as small as pins. The MP3 successor Advanced Audio Coding (AAC) is available/deployed nearly everywhere. Additionally, with MPEG-H 3D Audio a new audio codec is available and MPEG is working towards what is called MPEG-I Immersive Audio Coding, but that's another story..