Key moments from the Leno-Letterman-Carson late-night triangle

David Letterman was just a stand-up comic — albeit a successful one — when Johnny Carson took a shine to him. He often had Letterman as a guest on "The Tonight Show," and Letterman started serving as a regular guest host back in 1978.

Carson had been hosting "The Tonight Show" since 1962 when he negotiated a new deal in 1981. Needless to say, he had a lot of cache to get what he wanted. Carson chose to work three days a week, which led to an increased use of guest hosts. In 1983, this led to a series of "permanent" guest hosts who were used when Carson was off.

Letterman's success in stand-up and as guest host of "The Tonight Show," first resulted in him being given a morning show. It swiftly failed, but Letterman remained under contract with NBC. The network decided maybe the nighttime would work better for him. In 1982, "Late Night with David Letterman" debuted after Carson's "Tonight Show."

In September 1983, Joan Rivers was made the "official" permanent guest host of "The Tonight Show." She had this role for three years, and at the time Rivers would have been the favorite to replace Carson upon his retirement. Then in 1986, Rivers was offered her own talk show on the fledgling FOX network, which she accepted.

Rivers new show obviously ended her stint as guest host, but it ended so much more. Carson never forgave Rivers — feeling betrayed she would take her own show and compete against his. Rivers' show was quickly canceled, but she was never a guest on "The Tonight Show" during the Carson years again. In fact, Carson refused to even talk to her.

With Rivers out of the picture, the show needed new guest hosts. A couple of successful stand-ups took on the role. One was Garry Shandling, and the other was a guy by the name of Jay Leno. From 1987 through 1988, Shandling and Leno would both man the desk when Carson was on vacation or had the night off.

Being a late-night host didn't really strike Shandling's fancy. After a couple of years, Shandling left to focus on his groundbreaking Showtime series, "It's Garry Shandling's Show." Funny enough, Shandling would go on to play a talk show host in the iconic "Larry Sanders Show."

With Shandling out of the picture, the producers of "The Tonight Show" decided that Leno would shoulder the load as the only permanent guest host. He would serve this role from 1988 until 1992, when he would get a slightly larger one.

By the '90s, Carson was in his mid-60s, and he had been on TV for decades. He couldn't stick around forever, so Carson announced his retirement not just from "The Tonight Show" but essentially, from show business. For the first time since 1962, "The Tonight Show" would need a new host.

Unfortunately NBC did not have a succession plan in place. On the one hand you had Letterman, who was hosting the show that came on after Carson. On the other, there was Leno, who had been guest hosting "The Tonight Show" for years. Both men wanted the job, and the battle got ugly. The controversy was chronicled by Bill Carter in the 1994 book "The Late Shift," which was also turned into a movie by HBO.

In the end, NBC decided to go with Leno. This was despite the fact that Carson reportedly supported Letterman for the role. It has also been reported that Letterman didn't find out he didn't get the job until it was announced publicly.

Feeling resentful about losing out on "The Tonight Show" job, Letterman decided to leave NBC. Sensing opportunity, CBS offered Letterman his own late-night show. The network had tried its hand in the late-night game with "The Merv Griffin Show" and "The Pat Sajak Show" but was never able to compete with Carson. However, now Carson was gone, and the battle between Leno and Letterman would continue.

In truth, it's the penultimate "The Tonight Show Starring Johnny Carson" that people remember. That's the one with Robin Williams and Bette Midler. The real final episode, which aired May 22, 1992, featured no guests. It was a show looking back at Carson's long run at the desk complete with a heartfelt farewell. Carson's time as the king of late night was over.

The transition from Carson to Leno happened swiftly. On May 25, 1992, Leno hosted his first show as the official, one-and-only host of "The Tonight Show." His first episode featured Billy Crystal as a guest, but at the time there was no sense of how long Leno would be around or if he could replicate the success of his predecessor.

Without established infrastructure in place, it took a little time before CBS got Letterman's show off the ground. Finally on Aug. 30, 1993, "Late Show with David Letterman" debuted. It aired in the exact same time slot as "The Tonight Show," which meant Letterman and Leno were in direct competition. A late-night war for ratings had begun, with either Leno or Letterman being on top over the years.

Carson passed away on Jan. 23, 2005, when Letterman was unfortunately on vacation. When he returned, he did a monologue entirely from jokes he had gotten from Carson. It was revealed that throughout the years, Carson had been sending jokes to Letterman. If anybody was wondering which side of the Leno-Letterman debate he was on, the answer was suddenly clear.

Not wanting history to repeat itself, NBC got way out ahead of the succession plan for Leno. In 2004, the network announced that Conan O'Brien, who took over "Late Night" from Letterman, would take over for Leno in 2009. Then in 2008, it was decided that the final "Tonight Show with Jay Leno" would air May 29, 2009.

Unlike Carson, Leno didn't have any plans to retire. Instead, it was announced that he would host a nightly show on NBC at 10:00 PM ET, making him effectively Conan's lead-in. Leno hosted his last "Tonight Show" — O'Brien was a guest — and then began his new show in September of 2009.

Somehow, the Leno-O'Brien situation turned out even messier than Carson's retirement. It became clear that Leno never really wanted to retire and was itching to get back into his own role. There was talk of moving Leno's show to 11:35 and pushing "The Tonight Show" to 12:05. O'Brien refused, and a settlement was reached that let Conan out of his deal. His run as host of "The Tonight Show" didn't even make it a year.

Unsurprisingly, the Leno-Conan situation at NBC struck a chord with Letterman. During that time, he repeatedly discussed the matter on his own show — pulling no punches in his excoriation of Jay "Big Jaw" Leno. If anybody came out as a winner in this mess, it was probably Letterman, as Conan lost his show and Leno lost any goodwill he still had in the comedy community.

Leno took over "The Tonight Show" again on March 1, 2010, with NBC hoping the Winter Olympics would help boost his ratings. It did not work. By July, Leno's ratings were his worst since 1992. In October, Letterman overtook Leno in the ratings, though Leno regained the ratings lead in May 2011 and held onto it from then on. As for O'Brien, after touring during a period of time when he was legally prohibited from being on television, he started his own eponymous talk show on TBS. "Conan" remains on the air to this day.

Leno's second run as host of "The Tonight Show" turned out to be kind of a mess. NBC, dealing with money issues, laid off staff members left and right while rumors began swirling that Leno's time was running out. His contract was expiring in 2014, and it seemed unlikely he would last beyond that. Such predictions turned out to be correct. Leno's final "The Tonight Show" aired Feb. 6, 2014, paving the way for Jimmy Fallon to take over. The show returned to New York for the first time since Carson had it moved to Burbank. Leno has stayed retired — for now.

With Leno retired, Letterman had outlasted his longtime rival. However, he would not be long for television either. In April 2014, Letterman announced he would retire in 2015. Stephen Colbert was named his successor. The final episode of "Late Show with David Letterman" aired May 20, 2015. Letterman grew a beard and got himself a show on Netflix. He's never tried to take his show back from Colbert and has actually said on occasion he wishes he had retired sooner.