Why would folk be different?

Ours is an era in which pop music - as in, popular music; the stuff that gets traction, makes it to the radio playlists and the bigger venues - functions as a giant dressing-up box. A plaything in a happy attic where gilded children, unwitting of their material advantages and their creative deficits, knowing only that they really love music, come to rummage through the past. Try on this hat, that sound. Mummy, look. Daddy, look. BBC Sound Of list - look!

So why would folk be different? Why wouldn't it be something adopted as manner, a costume, by its most successful recent proponents? Mumford & Sons, Laura Marling, Noah And The Whale. A roster that runs from the fatuous to the insipid via the merely inoffensive.

Nothing new there, you might argue. The last century's custodians and revivalists tended not to be horny-handed sons and daughters of toil, either. What they had, however, was an idea of folk music not just a source, but as a living force, a humming thread that runs from the past through the present, and stretches unseen into the future.

The most inventive of today's British folk artists have this spirit about them, and I don't think it's a coincidence so many of them have recently been involved in historical narrative or documentary projects: Gruff Rhys' American Interior, King Creosote's From Scotland With Love, The Unthanks' Songs From The Shipyards. (This last, although first-rate, was a little out of character; social chronicles are not The Unthanks' specialty).

The Unthanks are the most overtly folkish of all of those acts, in a traditional sense, yet also one of the most discreetly radical things you may currently hear upon a stage. The beauty in their music - the limpid voices of Rachel and Becky Unthank; the slow and intricate delicacy of an-eight piece band (including string quartet) masterfully steered by keyboardist Adrian McNally - is obvious. Its wide-ranging ingenuity is less so. It creeps up on you on stockinged feet, one of those rare shows that somehow contrives to be both tasteful and daring. It swims in stillness. It's not so much haunting as haunted. The sorrows and frailties of centuries inhabit it.

That's no doubt why each half of their two-part set, largely drawn from their wonderful recent album Mount The Air, opens with the rambunctious humour of self-described "accordionist clown" Tim Dalling, and why they spend so much time gently joshing the crowd between numbers. (Only Lady Gaga talks more onstage, and the Unthank sisters are far better value on that score.) Any true entertainer knows you have to alternate your moods. Especially when you deal in such exquisite agonies as The Unthanks do. To be immersed in those without pause might be more than one's soul can withstand.

Like all the best sad music, it isn't remotely depressing. It simply takes you apart, softly, with immaculate, melancholy tenderness. That's the greatest power of folk's connection to the past, and what The Unthanks understand better than any group now drawing upon it: the human heart is changeless, its afflictions eternal. If yours isn't broken when you go in, it will be by the time you leave.