A listener asks: "What makes a concerto different from a suite? A bagatelle from a caprice? On my way to work once, Radio 3 Breakfast played a gentle, quiet piece, with chords languidly spread into arpeggios. Aha, I thought; this is a nocturne. But no, it was an etude."

So when is a song not a song? Tom Service tackles the complicated world of classical musical titles, catalogue numbers and naming conventions.