Walk into any high-street music store and you'll find music arranged according to genre. And as artists become ever more experimental, so the number of categories increases. This is nothing new. Back in the 1970s, East Coast hip-hop emerged as a sub-genre of hip-hop. Jamaican reggae and dancehall fused with Latin American influences to give reggaetón, a 1990s alternative to Britpop.
But some artists are becoming increasingly disgruntled at how their albums are being categorised by the stores - so much so, that they have started the "War in a Rack" campaign.
"Of course we need to compartmentalise music," says Soweto Kinch, a Dune Records artist whose music crosses the urban and jazz genres. "But where I'm racked has a huge effect on exposure." Filed under jazz but not urban (despite airplay on urban radio stations), Kinch is concerned. "Kids go to browse the hip-hop section, but how often do you see them go into the jazz department?"
It is not just about denying potential listeners; sales figures are affected and bad racking equals a low profile and less chance of scooping prizes. This year's Brit Awards, to be held on 14 February, back up Kinch's claim.
The nominations read like a catalogue of artists whose albums are stocked at the front of the shops: Lily Allen, Amy Winehouse, James Morrison, Kasabian. Even the Mobo and Nationwide Mercury awards, which usually champion less mainstream trends in music, are increasingly piggybacking on commercial success.
HMV explains that its racking system is based on the historical development of music and a sense of instinct. "Guidelines come from head office and are decided through a dialogue between the label and the executives. It comes down to common sense and instinct; these people have a lot of experience in the industry," says a spokesperson. But many artists disagree with this approach to deciding musical genres. In his book Arcana: musicians on music, John Zorn argues that such categorisation "commodifies and commercialises an artist's complex personal vision".
Fighters in The War in a Rack have taken this message to heart. Supporters complain to their local branches of Virgin and HMV and move records into appropriate categories. According to Kinch's blog: "Men in suits . . . are dictating to us what is or isn't hip-hop!"
The attack goes further. As work like his is denied exposure in the urban section, Kinch argues, "thousands of people are kept from seeing an alternative model of hip-hop with which they can identify. If Nelly Furtado and Justin Timberlake are urban, why is a hip-hop/jazz album set in a UK tower block not?"
He may have a point. Hip-hop culture is increasingly associated with youths in hoodies carrying guns, an image that some urban artists have done little to diminish. "Intellectual, left-wing" hip-hop - Kinch read history at Oxford and the newsreader Moira Stuart appears on one of his latest album's tracks - could help to alter the way in which street culture is viewed. It may be a fairer reflection of a way of life that is not just tied up with crime and antisocial behaviour.
But will the retail companies, driven by economic goals, take note of the requests from these more marginal artists?
"Musicians and buyers can always put forward good reasons for being racked elsewhere," says an HMV spokesperson. "At the end of the day, it simply comes down to customer service."