A jazz song composed with 7 tones to the octave, rather than the usual 12. It has a surprisingly mellow feel — the harmony is dissonant in a way our ears aren't used to, but it works well in a jazz context.

I definitely recommend checking out 5 hideya's other videos as well, especially this one where they write short snippets dividing the octave into every number from 1 to 11 notes.