Very sensible for, once you start, where do you end? Every time Myrone ventured out socially during the genesis of the exhibition, he found himself fielding a barrage of questions: "Will you be including tattoos, traveller art, my nan's knitting?"

Instead of attempting answers, Myrone and his colleagues Ruth Kenny and Jeff McMillan embarked on a country-wide rummage in museum vaults for objects that had already been labelled as "folk" by local curators. In practice, this often meant items that had arrived decades earlier that no one had ever quite known what to do with. The curators also rifled the Tate's own store-cupboard for items that appeared not to fit into the canonical categories by which we normally make sense of "art" (even the Tate has acquired its fair share of oddities over the decades).

An obvious example here is (1600-10), a painting of two gentlewomen executed not by a professional artist but by an artisan whose lack of formal technique results in a piece of work that is as vivid as it is puzzling. Are the women sisters, even twins? Why do they each have a baby? And why is the perspective so flat? Is it simply because the artist didn't know how to do depth?

The Tate's exhibition, which goes from right up to the mid-20th century, is part of a new interest in vernacular forms.

Almost a decade ago Jeremy Deller and Alan Kane's reframed Morris dancing, garage sales and cake-making as worthy of serious thought and a second look. And contemporary practitioners can't seem to get enough of the old ways of doing things. Grayson Perry broke through with pots, Tracey Emin makes quilts, while Bob and Roberta Smith (the pseudonym of artist Patrick Brill) has made painting signs his identifying mark.

Folk art is usually assumed to be anonymous, expressive of a whole culture rather than one person's intention and design. But, Myrone points out, it is impossible to imagine that you can discern a collective consciousness behind a body of work that just happens tobe unsigned.

In choosing which pieces to feature, the curators looked for the idiosyncrasies that suggested an individual imagination at work or play. Sorting through dozens of similar objects in the drawers and shelves of museum storerooms, it is the tobacco-shop highlander with the boss eyes or the sampler with the spelling mistake that have made the final cut.

The curators are also keen to smudge any easy ideas about which sex did what in the making of vernacular art. Some of the finest needlework in the show was produced by the butchest of men: between 1850 and 1910 recuperating soldiers were encouraged to cut up old serge and twill uniforms to make bright patchworks.

It's not for the faint of heart or fingers: the thickness and weight of the cloth meant piercing and sewing the quilts became the equivalent of an energetic march. That's why the government promoted the practice to draw soldiers away from liquor and dice. In 1875 monthly periodical published an article quoting a soldier who took up quilting when he gave up drinking: "I must be employed or I shall get into mischief." One of his patchwork quilts runs to more than 28,000 pieces.

Needlework wasn't used simply as a means of order and control. Soldiers and sailors were perfectly capable of subverting stitchwork to express their tenderest feelings. They often made exuberant heart-shaped pincushions to send to their girlfriends, extravagantly finished with fringing, sequins and pom-poms. One particularly ardent soul even used pearls to pick out the slogan "Remember Me" and dated it for good measure.

Other items in the exhibition seem almost designed to provoke a discussion about the nature and limits of folk art. Take the leather toby jugs. They follow the standard form of a seated figure in an oversized tricorn hat and knee breeches smoking a pipe. But over time the leather has distorted and buckled, so that the tobies have started to resemble squat and sinister gargoyles. The nails that hold them together are frequently machine-made and date from the early 20th century.

The best guess is that these unsettling objects are a joke from the fag-end of British industrialisation, a moment when some bright, ironic soul thought that it might be funny to mock the practice of exporting knock-off pottery toby jugs around the world as emblems of English good cheer.

As Myrone says, it is perhaps the ships' figureheads that ask the boldest questions about the border between artisanal and artistic production. In the 18th and 19th centuries, figureheads were made in highly organised workshops by skilled craftsmen. Yet there is usually an element of knock-about crudity to them: the busty floozy, the jolly tar, the moustachioed mandarin.

And then what happened when these massive structures were retired to dry land to become shop signs, memorials or even sculpture? Some of these works have even been provided with implausible legs on which to totter.

For Myrone the figureheads' slippery status - a strange word perhaps to use of something so hulking - makes them "the exemplary form of folk art".