The feud among Frank Zappa's heirs over the rights to their father's music, the filming of a documentary about his life and other matters escalated this week when Dweezil Zappa issued a multi-thousand-word rebuttal to his brother Ahmet's open letter to him on Facebook.

In what he characterized as "a tedious undertaking," Dweezil countered Ahmet's assertions that he was uncommunicative and intransigent and that he created a false impression of the family's disagreements by giving an "incomplete, misleading story" to The New York Times, in an article published last month that quoted both brothers.

The Zappa siblings -- Dweezil, Moon, Ahmet and Diva (but particularly Ahmet and Dweezil, who recorded two albums together) -- have been battling each other since their mother, Gail Zappa, died last year.

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Following their father's death from prostate cancer in 1993, the Zappa Family Trust was formed to administer the rights to Frank Zappa's vast catalog of songs and to protect copyrights and trademarks. After Gail's death last October, control of the trust passed to the younger Zappa siblings, Ahmet and Diva, who serve as the estate's trustees. Dweezil and Moon remain beneficiaries of the estate.

As reported in The New York Times, in April the Zappa Family Trust informed Dweezil that he did not have permission to perform any of his father's songs on his annual Zappa Plays Zappa tour; could no longer use the Zappa Plays Zappa name, which the trust owns and for which Dweezil said his mother had charged him an exorbitant licensing fee; and that he faced as much as $150,000 in copyright infringement each time he performed one of his father's songs.

"My father was Frank Zappa, but I am not allowed to use the name on its own," Dweezil told the Times.

"I am not standing in the way of Dweezil playing the music," Ahmet Zappa said. "He would just have to be in accordance with the family trust."

The Zappa siblings have also clashed over a yet-unfilmed, Kickstarter-funded documentary about their father, directed by Alex Winter, co-star of the Bill & Ted movies and more recently director of the Napster documentary Downloaded, which Ahmet and Diva endorse but Dweezil and Moon do not. (The project is unrelated to Eat That Question, a documentary comprised of old television interviews with the composer and guitarist, set for release June 14.)

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At the heart of the dispute is a contract between Dweezil and the Zappa Family Trust relating to the Zappa Plays Zappa tours.

"During her tenure as the executor of the ZFT, our mother created a Zappa Plays Zappa merchandise contract with me that she did not honor," Dweezil wrote in his rebuttal to Ahmet's letter.

Despite his objections, Dweezil alleges that, for a decade, Gail and the trust kept all the merchandising money from the tours. When control of the estate passed to Ahmet after her death and Dweezil informed his brother of the situation, "Instead of making it right, you demanded the same, to collect 100 percent of all monies from merchandise sold at my concerts even though I'm the one doing 100 percent of the work presenting our father's music to audiences around the world," Dweezil wrote.

Of Gail Zappa's famously litigious stewardship of the trust, Dwezil added, "In the name of protecting the music of our father, our mother created a legacy of her own. I'm not referring to that in a pleasant way." He cited "multi-million-dollar debt-inducing lawsuits, a determination to possess everything connected with our father (down to trademarking his mustache) and a perception that anyone playing Frank's music was somehow suspect and motivated by dubious intentions."

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Dweezil disputed Ahmet's assertion that he could continue touring under the Zappa Plays Zappa name upon paying a token $1 licensing fee because, Dweezil alleged, the arrangement was contingent on continuing to allow the trust to collect 100 percent of the tour merchandising revenue while not retroactively paying him his share dating to 2006 that he claimed he is owed.

Dweezil has since renamed the Zappa Plays Zappa tour Dweezil Zappa Plays Frank Zappa to "emancipate" himself from the estate, which Dweezil said Ahmet's lawyers claim "is illiquid and in millions of dollars of debt." In his letter to Dweezil, Ahmet said that maintaining the business side to their father's legacy was "pretty damn expensive…That's why Gail told us we have to sell the house: because she knew how much it would cost to maintain the catalog."

The "house" is the storied Zappa family compound in L.A.'s Laurel Canyon neighborhood, where Frank, Gail and the then-infant Moon moved in 1968 and where Frank composed and recorded much of his music. In March, the house was offered for sale for $9 million as part of the Kickstarter campaign to fund the Winter documentary and catalog Zappa’s fabled “Vault,” containing thousands of hours of unreleased recordings, video and family artifacts. (The crowd-funding campaign successfully met its goal of $1 million, and Winter is supervising the archiving and preservation of the Vault’s contents.) For the past month, moving vans have been seen coming and going from the property, and it now appears vacant.

If the immediate future, the Zappa Family Trust is in fact dependent upon the sale of the Zappa family compound -- other houses in the neighborhood have sold for between $2 and $4 million -- and keeping Frank Zappa's music relevant for future commercial exploitation would appear to be crucial for the trust's long-term viability, which Dweezil argues his Zappa Plays Zappa tour provides.

"The preservation of Frank's musical legacy is wholly dependent on the audience it achieves," Dweezil wrote to his brother. "No one else in our family has been performing this music for the past decade. No one else in the family has any ability to play any instrument on a professional level. No one else in the family has the ability to read charts or perform with the level of expertise required to perform this music commensurate with the standards our father set."

Alluding to his difficulties with the trust and addressing the brother and sister who control it, Dweezil added that restraining performances of their father's music "also jeopardizes the ability to create and sustain an audience. Our mother did not understand this concept and seemingly neither do you or Diva."