Fingerpickin' good: While country got rich, bluegrass stayed poor: Jasper Rees on the high speed sound of the white man's blues

In just about any American road movie you've ever seen, think back to that bit when they're in the soft-top saloon and driving across a sun-baked landscape in the middle of the continent. On the soundtrack there's either a harmonica, to denote slow progress, or there's a banjo, which means they're going like the clappers.

If any music is as evocative as the blues, then it's bluegrass: one's black and hugely popular, the other's white and an acquired taste, but they're both the poor man's invention and they've both had a massive say in the course of popular music. The biggest difference between them is that one went electric and progressed; the other kept its acoustic formula of fiddle, banjo, mandolin, bass and tenor harmonies, and sounds much the same as it did 60 years ago.

Actually bluegrass's most famous movie moment came not on the road but in John Boorman's Deliverance, in which one of the men on a fishing trip (played by Ronny Cox) and an inbred Appalachian boy duel on the banjo. The scene gives a sense of bluegrass's franticness, and also its roots in the back end of nowhere. Simple, often uneducated folk play this stuff, but they play it with an almost academic technical virtuosity.

'Duelling Banjos' as played by Eric Weissberg and Steve Mandel reached No 2 in America in 1973. 'The song has done a lot for the music,' says Nick Barraclough, who presents High and Lonesome, a six-part history of bluegrass starting tonight on Radio 2, 'but it's been a thorn in the side of every bluegrass band since, because to an uneducated audience it's the only thing they request.'

Bluegrass's other most famous tune is 'Blue Moon of Kentucky', which was covered by Elvis Presley but was written by the founding father of bluegrass, Bill Monroe. Monroe came of a large poor family in the Blue Ridge Mountains and took up the mandolin because his brothers had already nabbed the other traditional instruments. He struck out on his own in the late 1930s, by which time he had started writing his own songs. His band, the Bluegrass Boys, developed a high-speed country style tinged with polka and Scottish reels and became the finishing school for all bluegrass musicians of any note - all, that is, except mandolinists. The band even gave its name to the music, when people asked for 'some more of that Bluegrass stuff'.

Monroe's influence on bluegrass cannot be overestimated. He's still the only bluegrass musician to have consistently made a decent living. There are many reasons why bluegrass is not the commercial force it could be, but the main one is that it places relatively minor stress on composition.

'It is such a regimented music that not only do you have to have the right instrumentation but the guitar must be a Martin D28, the mandolin has to be a Lloyd Lore Gibson F5 and the banjo has to be a Gibson Mastertone,' says Barraclough. 'You've got to have that model or the sound would just not quite be right.'

Like the boy in Deliverance, bluegrass is inbred. A lot of the best acts - the Stanley Brothers, the Osborne Brothers, Jim and Jesse - come from close-knit families. This is music that is turned in on itself, though other musicians are always peering over its shoulder.

The other thing, of course, is that bluegrass is neither trendy nor politically correct. Monroe may have had wines and girlfriends galore, but there was only ever one woman in his band, a bass player called Bessie Lee Maudlin, who stood at the back.

Now women are fronting bands, and Barraclough nominates a young songwriter called Alison Krauss as the best chance for bluegrass. She sold 100,000 albums last year. But most bluegrass musicians couldn't afford to dress as vulgarly as their Country cousins even if they wanted to. 'There's no swagger in bluegrass, no airs and graces,' says Barraclough. 'They're not rich, but they're revered.'

(Photograph omitted)