Tom Service savours the sound of the fifth - an interval with many meanings, from mystic drone to military bugle call. He's joined by Early Music expert Jeremy Llewellyn who explains the significance of the fifth in medieval music, related to The Music of the Spheres and used to invoke the Almighty in religious chant; and by composer David Bruce, who describes how composers today find fresh uses for this primal sound. Tom finds the open, ringing sound of the fifth in all sorts of music, from a Buzzcocks guitar solo to a Bruckner symphony, providing the thrill of adventure in the Star Wars theme and underpinning the reels of Scottish bagpipe music.