Oral Tradition

The idea that nothing is written down, but passed on by word of mouth. This applies to stories and songs more than tunes which are more of a aural tradition. Hear and repeat.

In essence, writing down the words of a song or the notes of a tune freeze it forever. There will always be somebody afterwards who says that's not how it's written.

If Folk means anything, it means never having to say "that's not how it's written".