The song may sound familiar at first, thanks to the unmistakable guitar riff from Nirvana's classic "Smells Like Teen Spirit."

But suddenly, the recording changes course when instead of the gravelly voice of Kurt Cobain, the smooth R&B harmonies from a Destiny's Child hit appear on top of the grunge instrumental. As the recording moves on, it is clear that the song is neither fish nor fowl; it is a crossbreed that neither band ever intended or even dreamed of.

It is something that is completely different, generally illegal and, thanks to the Internet, becoming explosively popular.

Songs like this one, which combine different hits without adding any original music, may represent the first significant new musical genre to be lifted out of the underground, developed and then spread, all via the Web. The songs, called mash-ups or bootlegs, typically match the rhythm, melody and underlying spirit of the instrumentals of one song with the a cappella vocals of another. And the more odd the pairing the better.

The music industry has greeted them with mixed response. A radio station in London playing a popular mash-up with Christina Aguilera belting her hit "Genie in a Bottle" over the retro-rock of the Strokes was served with a cease-and-desist order by Aguilera's publisher, Warner/Chappell.

On the other hand, in Britain last week, Island Records released a legal mash-up, which entered the pop charts at No. 1. It combines music from three artists — the new-wave icon Gary Newman, the R&B singer Adina Howard and the girl-pop group the Sugababes.

The music — there are hundreds of such recordings — is particularly popular in Europe. But through the Internet it is spreading to the United States. There are so many bootlegs using Eminem and Missy Elliott songs (Eminem mixed with the fey pop of the Smiths, and Elliott mixed with the heavy metal group Metallica, for starters) that some practitioners refer to making a bootleg as "doing a Missy" on a song.

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Naturally, the music industry is concerned about this, because in most cases the tracks are being used without artists' permission.

But today, when the Internet seems to loom larger in many music fans' heads than lawyers' threats, bedroom musicians on both sides of the Atlantic are undeterred. All they need to do is download or buy software programs like Acid (which automatically synchronizes the rhythms of different tracks). Then they can scour a file-sharing service for a cappella versions of songs, which record companies sometimes include on promotional singles for club disc jockeys. Then, using a program like Acid, they can combine the material into a new song.

The mark of a good bootleg, fans say, is that it doesn't sound like one song superimposed on top of another, but a new song in itself.

"If you take two or three or four great records and mix them together, you should end up with a superior product," said Steve Mannion, a co-editor at Boom Selection www.base58.com, a Web site dedicated to documenting the do-it-yourself remix, bootleg and sampling movements. "The best bootlegs don't sound like bootlegs; they work at a profound level and actually sound like they are the original record."

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