From medieval English music to the Everly Brothers - what is it about the musical interval of the third that sounds so attractive? Why does a major third tend to feel positive, and a minor third tend to feel sad? Nature or nurture? And what about their dark cousin, the tritone - the so-called "Devil in Music" - what on earth can be that sinister about a couple of notes?

Tom Service is joined by Dr Adam Ockelford to try and find some answers.