Speculations and specifications. If you were a Winamp user back in the day, or curate an MP3 collection currently, you might recognize the humble ID3 tag. It is what the metadata in the MP3 file is made up of. First it was pretty limited in the version later dubbed ID3v1. Like any good 2.0 they added a ton more fields, features, removed character limits and it was suddenly ID3v2. The latest spec is ID3v2.4 while the most commonly adopted one seems to be ID3v2.3. I recently found myself having a reason to dig into this specification.