When I was a kid I had a big box of C15 tapes. The number is the length in minutes. 15 minutes isn't all that useful for music or voice, but it was ideal for ZX Spectrum and Commodore 64 games (and saves, and software I wrote myself.) I also had a reporter's notebook with the tape name and tape counter numbers for when things I'd saved started. They were very easy to copy and quick cheap.