The Internet put the music industry and many of its listeners at odds thanks to the popularity of services like Napster and Grokster. Now the industry is squaring off against a surprising new opponent: musicians.

In the last few months, trade groups representing music publishers have used the threat of copyright lawsuits to shut down guitar tablature sites, where users exchange tips on how to play songs like “Knockin’ on Heaven’s Door,” “Highway to Hell” and thousands of others.

The battle shares many similarities with the war between Napster and the music recording industry, but this time it involves free sites like Olga.net, GuitarTabs.com and MyGuitarTabs.com and even discussion boards on the Google Groups service like alt.guitar.tab and rec.music.makers.guitar.tablature, where amateur musicians trade “tabs” — music notation especially for guitar — for songs they have figured out or have copied from music books.

On the other side are music publishers like Sony/ATV, which holds the rights to the songs of John Mayer, and EMI, which publishes Christina Aguilera’s music.

“People can get it for free on the Internet, and it’s hurting the songwriters,” said Lauren Keiser, who is president of the Music Publishers’ Association and chief executive of Carl Fischer, a music publisher in New York.

So far, the Music Publishers’ Association and the National Music Publishers’ Association have shut down several Web sites, or have pressured them to remove all of their tabs, but users have quickly migrated to other sites. According to comScore Media Metrix, an Internet statistics service, Ultimate-Guitar.com had 1.4 million visitors in July, twice the number from a year earlier.

The publishers, who share royalties with composers each time customers buy sheet music or books of guitar tablature, maintain that tablature postings, even inaccurate ones, are protected by copyright laws because the postings represent “derivative works” related to the original compositions, to use the industry jargon.

The publishers told the sites that if they did not remove the tablatures, they could face legal action or their Internet service providers would be pressured to shut down their sites. All of the sites have taken down their tabs voluntarily, but grudgingly.

The tablature sites argue that they are merely conduits for an online discussion about guitar techniques, and that their services help the industry.

“The publishers can’t dispute the fact that the popularity of playing guitar has exploded because of sites like mine,” said Robert Balch, the publisher of Guitar Tab Universe (guitartabs.cc), in Los Angeles. “And any person that buys a guitar book during their lifetime, that money goes to the publishers.”

Mr. Balch, who took down guitar tabs from his site in late July at the behest of the music publishers, added that, “I’d think the music publishers would be happy to have sites that get people interested in becoming one of their customers.”

Cathal Woods, who manages Olga.net, one of the pioneer free tablature sites, said he had run the site for 14 years with the help of a systems administrator, “and we’ve never taken a penny.” Mr. Woods, who teaches philosophy at Virginia Wesleyan College in Norfolk, said Olga.net had earned an undisclosed amount of money by posting ads on Google’s behalf, but he said that money had paid for bandwidth and a legal defense fund.

Anthony DeGidio, a lawyer for Olga.net, said he was still formulating a legal strategy, while also helping decide whether the site could pay licensing fees “in the event that that’s required.” For now, though, the site remains unavailable to users.

Because the music tablature sites are privately held, they do not disclose sales figures, and because industry analysts generally do not closely follow tablature sites, it is unclear how much revenue they generate. But with the Internet advertising market surging, almost any Web site with significant traffic can generate revenue.

Google also dabbles in tablature through its Google Groups discussion board service, in which guitar players trade tabs they have figured out by listening to the songs, or by copying tabs found elsewhere. A Google spokesman, Steve Langdon, said Google would take down music tablature from its Groups service if publishers claimed the materials violated copyright agreements and if Google determined that infringement was likely. Under the Digital Millennium Copyright Act, Web hosts may review, case by case, a publisher’s claims regarding instances of copyright infringement.

To hear music publishers tell it, though, the tablature sites are getting away with mass theft. Mr. Keiser, of the Music Publishers’ Association, said that before these sites started operating in the early ‘90s, the most popular printed tablatures typically sold 25,000 copies in a year. Now the most popular sell 5,000 copies at most.

But Mike Happoldt, who was a member of the ’90’s band Sublime and whose music is sold in sheet music books, said he sympathized with the tablature sites.

“I think this is greed on the publishers’ parts,” said Mr. Happoldt, who played guitar on Sublime’s hit “What I Got.”

“I guess in a way I might be losing money from these sites, but as a musician I look at it more as a service,” said Mr. Happoldt, who now owns an independent record company, Skunk Records. “And really, those books just don’t sell that much for most people.”

Assuming a tablature site musters the legal resources to challenge the publishers in court, some legal scholars say they believe publishers may have difficulty arguing their complaints successfully. Jonathan Zittrain, the professor of Internet governance and regulation at Oxford University, said “it isn’t at all clear” that the publishers’ claim would succeed because no court doctrine has been written on guitar tablature.

Mr. Zittrain said the tablature sites could well have a free speech defense. But because the Supreme Court, in a 2003 case involving the extension of copyright terms, declined to determine when overenforcement or interpretation of copyright might raise a free speech problem, the success of that argument was questionable. “It’s possible, though, that this is one reason why guitar tabs generated by people would be found to fit fair use,” Mr. Zittrain said, “or would be found not to be a derivative work to begin with.”

Doug Osborn, an executive vice president with Ultimate-Guitar.com said his site violated no laws because its headquarters were in Russia, and the site’s practices complied with Russian laws.

Jacqueline C. Charlesworth, senior vice president and general counsel of the National Music Publishers’ Association, would not comment on the legality of specific sites, including Ultimate-Guitar, but she said she had seen no international licensing agreements that might make free United States distribution of guitar tablature legal.

Online discussion boards have been thick with comments from guitar tablature fans, looking for sites that are still operating and lamenting the fate of sites they frequented. One user of the guitarnoise.com forums, who calls himself “the dali lima,” said he had no doubt that the music publishers would win the battle.

“Hopefully we will get to a place where the sheet music/tab will be available online just like music — $0.99 a song. The ironic thing might be that a service like that — with fully licensed music/tab offered at a low per song rate — might actually benefit guitar players by providing the correct music/tab and not the garbage that we currently sift through.”

A small handful of sheet music sites now sell guitar tablature. Mr. Keiser, of the Music Publishers’ Association, estimated that, including overhead costs, tablature could cost about $800 per song to produce, license and format for downloading.

Musicnotes, an online sheet music business based in Madison, Wis., is considering a deeper push into guitar tablature, said Tim Reiland, the company’s chairman and chief financial officer. The site has a limited array of tablature available now for about $5 a song, and it also offers tablature as part of $10 downloadable guitar lessons.

But Mr. Reiland said that with the music publishers “dealing with the free sites,” and a stronger ad market, his business might be able to lower the cost of its guitar tabs.

“Maybe we could sell some of the riffs to Jimmy Page’s solo in ‘Stairway to Heaven’ for a buck, since that’s really what the kids want to learn anyway,” Mr. Reiland said.

Low prices are only part of the battle, though, Mr. Reiland said. The free tablature sites often host vibrant communities of musicians, who rate each other’s tablature and trade ideas and commentary, and Musicnotes would have to find a way to replicate that environment on its site. Furthermore, these communities often create tablature for songs that have little or no commercial value, he said.

“Less than 25 percent of the music out there ends up in sheet music because sometimes it just doesn’t pay to do it,” Mr. Reiland said. “So the fact that someone comes up with a transcription themselves just because they love that song and want to share it with people, there’s some value to that.”

“I don’t have an answer for that,” Mr. Reiland added. “But I think the industry needs to play around with it, because it could be a nice source of revenue for songwriters, and for the community it could be a really good thing.”

They should put the tabs right on the CD, with a "lock" on the info. To get the key, you'd go to a website and pay a dollar, or something like that, to unlock the file. They'd sell a lot of tabs and CDs that way.