The untimely death of Eagles founding member Glenn Frey on Jan. 18 at 67 could well close the curtain on one of the most remarkable touring careers in history.

The Eagles' touring history, marked by a 14-year hiatus after they disbanded in 1980 until they returned in 1994 and through their most current (and biggest) tour in History of the Eagles, has impacted the live industry both in the unprecedented business generated and the strategy behind it, engineered by longtime manager Irving Azoff -- and, to no small degree, Frey himself.

Formed in Southern California in 1971 from the nucleus of Linda Rondstadt's backing band, the Eagles quickly developed into a hard-touring act playing radio-friendly hits, and were headlining arenas by the time they were touring behind their fourth album, the hit-laden One Of These Nights, in 1975. "The Eagles were always great live," Azoff tells Billboard. "They learned their craft and their work ethic from the very start. It was like one big frat party out there. Who wouldn't want to do that? They were single rock stars during the '70s."

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The Eagles' recording career was a force on the Billboard charts, notching six No. 1 albums, including their last, 2007's Long Road Out of Eden. The Eagles' Their Greatest Hits 1971-1975, which topped the chart for five weeks in 1976, is the second-best-selling album all-time, according to the Recording Industry Assn. of America (RIAA), with sales of 29 million; only Michael Jackson's Thriller (30 million) is certified as having sold more. On the Billboard Hot 100, the Eagles scored five No. 1 hits, 10 Hot 100 top 10s and 21 Hot 100 hits overall. Those songs, most of them co-written by Frey and Eagles co-founder Don Henley, have been, and will continue to be, enjoyed by millions of fans, but that part of the career largely came to an end with The Long Run, with only one new studio album released by the Eagles in the past 35 years.

The final show from the Eagles' Long Run tour was March 4, 1980, at the Forum in Inglewood, Calif., though the band did reconvene for a political benefit to re-elect California Sen. Alan Cranston at the Long Beach (Calif.) Arena. The show, dubbed "Long Night in Wrong Beach" by the band, turned out to be what they thought was their last, with Henley famously saying the band would play together again "when Hell freezes over." Fourteen years later, they reunited for the (mostly) live album Hell Freezes Over, recorded April 24-25 at Warner Bros. Studios Sound Stage in Burbank, Calif. Frey famously joked on the album, and throughout that tour, that the Eagles "never broke up; we just took a 14-year vacation."

If that's the case, the band returned to work with a vengeance. The first rehearsals were "like a college reunion," Azoff recalls. "Fourteen years apart, but in a couple of days they were playing better than their first time around. The Hell Freezes Over tour, and the subsequent ones, were musically better than the first time around. The band was more focused, healthier, [there was] better technology etc."

At first, the rehearsals for Hell Freezes Over were "frustrating," as Joe Walsh puts it in the History of the Eagles documentary. "We were rusty," Walsh says, adding, "We really wanted to do it. We were eager to get our chops back up and show that we weren't has-beens, and we could still rock. It's not like it was 14 years. It's like we took a week off or something. I don't get it."

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The Hell Freezes Over tour of 1994-96 remains one of the most successful tours ever, moving 3.4 million tickets at arenas, amphitheaters and stadiums in North America, the U.K/Europe, Australia and Japan, according to Billboard Boxscore. "Hell Freezes Over was originally planned as a six-week tour to see how it would go," says Azoff. "Twenty-two years later [the reunion] was still going strong. The Eagles claimed their rightful place as America's biggest band ever."

A consistent, superstar touring act before the breakup (the Eagles Live album recorded on their Long Run tour of 1980 is certified at 3.5 million copies by the RIAA), the Eagles' post-reunion global touring career placed them among the elite touring acts of all-time. In addition to reuniting the Eagles, Hell Freezes Over also boasts another distinction: the tour heralded a new era in ticket pricing for elite touring acts. The Eagles were the first rock band to shatter the $100 ceiling for tickets. Azoff pointed out at the time that the only people who complained were journo types who got their tickets for free, and it's true that fans didn't balk, with Hell Freezes Over selling out every show.

Twenty-two years later, Azoff stands behind that pricing strategy. "Tickets should be priced at what the fans are willing to pay to see the performer," Azoff explains. "The key element in breaking the $100 ceiling was that it changed the dynamic of resale [by] having the revenue generated by tickets going back to the business -- promoters, artists, venues, etc." Clearly, demand to see the Eagles live after 14 years was sky high, but, "it wasn't just about making more money," Azoff says. "It was also about sending the message that Eagles were America's biggest band, and perhaps one of the biggest in the world -- charging like it influenced both fans and media that we were the biggest ever."

After playing intermittently as the millennium wound down, the group resumed touring full-time in 2001, with a line-up consisting of Frey, Henley, Joe Walsh, and Timothy B. Schmit, along with Steuart Smith and other sidemen. In 2007, the Eagles released Long Road Out of Eden, their first studio album in 28 years and their sixth number one album. The next year, on March 20, 2008, the Eagles launched their world tour in support of Eden at the O2 Arena in London. According to information supplied by the band, the Eagles played 228 shows from 2001 through 2013 (including a 15-date Australian run in 2004 and a North American stadium tour in the summer of 2010 with the Dixie Chicks and Keith Urban), much of it under the Farewell, Part I banner. On June 14, 2005, the Eagles released a new 2-DVD set, Farewell 1 Tour-Live from Melbourne.

Presumably making a joke out of the endless "farewell" tours common among classic rock acts, calling the tour "Farewell, Part I" exemplifies the sense of humor the Eagles displayed throughout their career. Even so, the Eagles, "took themselves very seriously," Azoff says. "They always understood the Eagles' legacy, and took the responsibility of preserving and protecting that legacy very seriously. As Glenn often said, it was about 'song power.' Their ability to invent phrases and tongue in cheek sayings was so clever, it was part of the mystique."

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In 2013, the Eagles began the extended History of the Eagles Tour in conjunction with the band's documentary release, History of the Eagles, which aired on Showtime prior to its DVD issue on Universal, followed by an exclusive on Netflix. Azoff says Frey was the "driving force" behind the film, a compelling, no-holds-barred look back, directed by Alison Ellwood and produced by Alex Gibney, that chronicles the band's rise, inner turmoil, 14-year split, and triumphant return in Hell Freezes Over. "[Frey] really sat down with me and planned it, plotted it and executed it," Azoff says.

Though it rarely was recognized as such, touring behind a documentary, using channels and creating new ones that historically had been put into gear for album releases, was a groundbreaking move, accomplishing "way more for them at this point in their career than any album could have done," Azoff said in an earlier interview. In terms of how to maximize creative content, History is a prime example of innovative new business models, on a par with the Eagles' Wal-Mart exclusive on Long Road Out of Eden, a move that received much more attention.

That's fine with the band, who were "never about recognizing their accomplishments," Azoff says. "It was always about presenting their fans with the best possible show. History of the Eagles was an amazing film. Yes, it had a huge impact, and the last legs of the tour were the highest -grossing ever. We didn't win awards, but we consistently won the battle of word-of-mouth. Netflix and Showtime say it has been viewed by amazing numbers of people."

Demand to see the Eagles live has never waned. The History of the Eagles tour -- presumably their last -- grossed $253 million and sold over 2 million tickets to 147 shows, according to a source with the band. Since reuniting in 1994, the Eagles have reported 564 shows totaling $850 million in gross and 8.8 million tickets sold; given that the Boxscore total is short by approximately one-third, it is safe to assume that the Eagles have grossed over $1 billion and been seen by 10 million fans worldwide.

Asked if that estimate is in the ballpark, Azoff says, "I'm sure it was a billion," adding that the band's merch numbers were equally strong. "Eagles most likely sold more merch than any other touring band during that period. Merch per caps oftentimes set building records. I'm sure it exceeded a billion [dollars]."

In the end, the Eagles leave behind a touring legacy unique in live music history as the most successful band reunion of all time. Beyond that, "They gave the fans what they wanted, with incredible sound, production, and flawless instrumental and vocal performances," Azoff says. "If you wanted to hear your favorites like the record, there was no one better…..ever."

Azoff declines to address the future of the Eagles as a touring entity. As to what managing the band has meant to him personally and professionally, Azoff says, "I am among the very few that surpassed the American dream. None of it was possible without Don [Henley], Glenn and the Eagles. Nothing else I've done even scratches the surface to the gratification of the trust they put in me, and what they accomplished, and allowed me to be part of."

CORRECTION: An earlier version of this article mistakenly listed sales of the Eagles Live album as 35 million copies. It has sold 3.5 million, according to the RIAA.