How do you measure the impact of someone who changed late-night television? Simply put, you can’t. But you can chart the most important thing that happened to him every year of his career. Just before David Letterman signs off “Late Show” for good on Wednesday, here are the most significant highlights (and a couple lowlights) of the host’s 33 years on late-night TV.

1982: “Late Night With David Letterman” debuts on NBC. (Feb. 1)

In 1980, Letterman, 33, was a breakout stand-up comic from Indiana when NBC gave him a live morning show, “The David Letterman Show.” Though critics loved the niche comedy, the ratings were so low that NBC was forced to cancel it after six months. But the network realized they had a star, so eventually executives let Letterman have his own late-night program, airing after “The Tonight Show With Johnny Carson.” That was the birth of “Late Night” on NBC.

The show started out emphasizing Letterman’s unique brand of comedy: The first face viewers saw was Larry “Bud” Melman doing a “Frankenstein” parody, and the year’s highlight was a bizarre, staged argument between comic Andy Kaufman and wrestler Jerry Lawler.

“Letterman looked a bit spooked as he walked onto the stage at NBC’s studio 6A for the first time,” Bill Carter wrote in his 1994 book “The Late Shift.” “[Inaugural guest] Bill Murray filled most of the hour ranting around the stage doing calisthenics while singing ‘Physical’ in a bad lounge-singer voice.”

1983: The first “Stupid Human Tricks” segment premieres.
(Oct. 3)

The now-famous bit — where you might see someone, say, playing a violin on a pogo stick — was born out of a similar one, the self-explanatory “Stupid Pet Tricks,” which debuted on Letterman’s morning show. It was the brainchild of Merrill Markoe, the first head writer on “Late Night” and Letterman’s longtime girlfriend. According to Carter, Markoe came up with the idea because she and Letterman were fairly obsessed with their dog, Bob.

1984: The suit of Velcro. (Feb. 28)

Letterman had a penchant for wearing odd suits, particularly his famous suit of Velcro. He would jump on a trampoline and fly toward a Velcro wall — and literally stick the landing. Other favorite suits included a suit of sponges, in which he got dunked in a 1,000-gallon tank of water; a suit of magnets, in which he attached himself to a refrigerator; and a suit of Rice Krispies, in which he snapped, crackled and popped in a tub of milk.

The peculiar bit signified how different Letterman was from other talk shows on air and why younger viewers — particularly college students — flocked to “Late Night.” Within a few years, Carter wrote, Letterman was bringing in approximately $20 million for NBC.

“They never set out deliberately to parody the conventional talk show; they only tried to be different,” Carter wrote of Letterman’s act, which had a “post-sixties ironic sensibility.” “But the result was something like the reverse image of a talk show.”

1985: The first Top 10 list.
(Sept. 18)

“Top 10 Things That Almost Rhyme With Peas” kicked off decades of Top 10s, mostly delivered by Letterman but sometimes by movie stars, sports stars and presidents. According to the Chicago Tribune, former “Late Night” writer Steve O’Donnell came up with the idea after seeing Cosmopolitan list octogenarian William Paley as a top-10 most-eligible bachelor; he decided “Late Night” might as well also write nonsensical lists.

1986: Cher calls Letterman an “a–hole.” (May 22)

Cher is certainly not the only celebrity to take issue with Letterman’s prickly personality, but she’s probably the best-known example of someone saying it to his face. “I knew you probably felt that way,” Letterman acknowledged during their interview, in which they explored why Cher originally didn’t want to appear on the program. “I think a lot of people feel that way about me, though.” (The next year, Cher reunited with Sonny on the show to sing “I Got You Babe.”)

1987: Letterman walks off during the Crispin Glover interview. (July 28)

The “Back to the Future” actor behaved so aggressively strange that Letterman abruptly departed during their chat. “I’m gonna go check on the Top 10,” he said.

1988: Harvey Pekar gets banned when Letterman gets angry. (Aug. 31)

Pekar got on Letterman’s nerves for the final time when he said Letterman looked like a “shill” for GE. Afterward, the “American Splendor” comic-book writer was banned for life.

1989: Oprah Winfrey’s first appearance starts their feud. (May 2)

This awkward interview between Oprah and Letterman was the first incident in a famed feud that lasted 16 years until Oprah’s next appearance on the show in 2005. Apparently, Oprah was mad that Letterman didn’t defend her from an audience member who was upset about a controversial episode of her daytime talk show.

1990: Gary Busey makes a strange appearance, and Glover shows up again.

These wild and unpredictable appearances helped establish that there was no one on TV better than Letterman at presiding over uncomfortable situations.

1991: Johnny Carson publicly announces his retirement. (May 21)

…in front of some very surprised NBC executives. Immediately, the rumor mill started buzzing about who would replace him a year later.

Given that Carson adored Letterman, who had the slot after him, it was widely assumed that he would take the “Tonight Show” throne. However, after some complicated backroom shenanigans, Jay Leno got the job, shocking everyone and leading up to a fraught era known as “the late-night wars.” (Three years later, Carson’s final TV appearance would come on Letterman’s show, not Leno’s.)

1992: Carson’s last show. (May 22)

Leno officially took the “Tonight Show” reins on May 25. Letterman, deeply sarcastic and never one to show any emotion, by all accounts was deeply stung that he was passed over for the job.

1993: The final “Late Night With David Letterman” on NBC airs, and the first episode of “Late Show With David Letterman” airs on CBS. (June 25 and Aug. 30)

Obviously, Letterman’s relationship with NBC executives quickly soured, and CBS was more than happy
to step in and offer him a better slot in the late-night game. Eventually, he accepted the offer and started a historic era of the “Late Show” — and his fans followed.

On May 20, David Letterman hosted his last show, ending a 33-year career on late-night television. Here are some of his top moments from CBS's "The Late Show with David Letterman" and NBC's "Late Night." (Jayne W. Orenstein/The Washington Post)

1994: Madonna drops 14 F-bombs. (March 31)

Apparently, Madonna wasn’t too pleased that Letterman introduced her as a world-famous pop star who “slept with some of the biggest names in the entertainment industry.” Madonna responded to this with an expletive-and innuendo-filled interview that, naturally, did gangbuster ratings.

1995: Drew Barrymore flashes Letterman for his birthday. (April 12)

Maybe one of the most famous moments in late-night history. That night sure beat the one he had a couple of weeks earlier, when he famously bombed hosting the Academy Awards. ( “Oprah. Uma. Oprah. Uma.”) But Dave had built up so much goodwill that it didn’t really hurt him in the long run.

1996: “Late Show” tinkers with a commercial-free format. (Sept. 20)

After a rocky start, Jay Leno’s “Tonight Show” gained steam thanks to a now-famous Hugh Grant interview in 1995. CBS tried a handful of gimmicks to keep Letterman’s ratings at the top.

1997: Farrah Fawcett’s loopiest interview ever. (June 5)

Letterman holds his ground as Fawcett rambles on incoherently, while everyone watching wonders the same thing: Uh, what the heck is wrong with her?

1998: New York Yankees pitcher David Wells stops by after pitching the 15th perfect game in baseball history.
(May 17)

A classic interview for the “Late Show,” which eventually grows to symbolize all things New York.

1999: John Malkovich reads the Top 10 list. (Oct. 15)

The “Top 10 Things That Sound Creepy When Said by John Malkovich” — this was the same year “Being John Malkovich” was in theaters — served as a memorable reminder of the power of Top 10 lists.

2000: The heart surgery episode. (Feb. 21)

Letterman kept things light after taking a five-week break to undergo quintuple bypass surgery. “My career flashed before my eyes — and I’ll tell you something, it was mostly awkward silences,” Letterman joked during his monologue. Later, Robin Williams appeared in surgical scrubs. However, Letterman briefly got serious when he invited his doctors and nurses onstage, thanking them for saving his life.

2001: The 9/11 monologue.
(Sept. 17)

Letterman’s heartfelt and riveting opening to his first show back after the 2001 terrorist attacks is largely thought of as the model for how any comedian should handle tragedy. Later, Dan Rather broke down in tears twice during his interview.

2002: The Warren Zevon episode.
(Oct. 30)

Letterman devoted the entire episode to a striking appearance by Zevon, one of Letterman’s favorite musicians and a frequent guest who had been diagnosed with terminal lung cancer and had only months to live. Zevon’s famous advice about life from the episode? “Enjoy every sandwich.”

2003: Harry Letterman is born. (Nov. 3)

Curmudgeonly Dave turned into Dave the Dad when he became a father for the first time at age 56, and delighted in telling stories about fatherhood.

2004: Janet Jackson’s post-Super Bowl interview. (March 29)

Hmm, what did Jackson think was going to happen when she appeared on Letterman’s show right after that infamous wardrobe malfunction? Letterman grilled her about the incident while Jackson demurred, saying, “I don’t want to relive any of that.” “All right,” Letterman responded. “You don’t mind if I ask you some questions about it, though?”

Jackson squirmed and reiterated that her exposure was truly an accident: “It wasn’t supposed to happen the way it did, Dave.” Letterman speculates: “That’s why I believe it was this Timberlake guy.”

2005: Johnny Carson tribute. (Jan. 31)

After Carson’s death, Letterman pays his idol a touching tribute, talking about his impact on millions: “At the end of the day, that’s who you wanted to be there.”

2006: Contentious Bill O’Reilly interviews.
(Oct. 27)

Letterman proved he was no O’Reilly fan, calling him a “bonehead” at one point when the Fox News host appeared in October (after a previous sparring match in January). “You’re putting words in my mouth,” he told O’Reilly as they argued about the Iraq war, “just the way you put artificial facts in your head.”

2007: The Paris Hilton post-jail interview. (Sept. 28)

Letterman cements his status as an interviewer who won’t mess around with fluff questions, bluntly asking the clearly uncomfortable reality star, “How’d you like being in jail?” A nation tired of “The Simple Life” cheered.

2008: The John McCain feud.
(Oct. 16)

Oops. Presidential candidate McCain bailed on the “Late Show” because he had to run back to Washington to deal with the economic crisis. No, wait, he just had to do an interview on the “CBS Evening News” with Katie Couric. Letterman was merciless when he caught McCain (on camera) in the fib, roasting him so much that McCain eventually agreed to rearrange his schedule and appear on the show.

2009: The sex scandal. (Oct. 1)

Letterman made a stunning confession during a taping: A CBS employee had tried to blackmail him after finding out that Letterman had slept with women on his staff. It turns out the employee was “48 Hours” producer Robert Halderman, who was dating Stephanie Birkitt, who was Letterman’s former assistant and with whom Letterman had an affair. Letterman wasn’t joking — Halderman was eventually sentenced to six months in prison for his scheme and America was riveted by the scandal. In an unfortunate coincidence, this all transpired about six months after Letterman married his longtime girlfriend, Regina Lasko, whom he apologized to on the air.

2010: The other late-night war. (Jan. 20)

There’s a late-night controversy that doesn’t involve Letterman, as Conan O’Brien steps down from “The Tonight Show” and Leno happily returns to take back the chair after leaving the previous year. Letterman delighted in skewering his old nemesis Leno for returning to the gig and mocked Leno for telling people not to blame Conan for the mess. Uh, who would possibly blame Conan? “I know a lot of you people think Conan pushed himself out of a job,” Letterman said faux-seriously. “He’s not that kind of guy. He would never do that to himself.”

2011: Lady Gaga eats the paper with the interview questions.
(May 17)

That’s pretty much what happened.

2012: The Hurricane Sandy episode. (Oct. 29)

In the aftermath of Hurricane Sandy, Letterman aired a show without a studio audience. It inadvertently reminded some people of the old days when Letterman would make things up as he went along. Also in 2012: Conan stopped by the show and the pair bonded over the late-night “Tonight Show” madness. “I was delighted by everything that happened, except you losing your job,” Letterman said. “The only consolation I took during that period is that you were happy,” Conan deadpanned.

2013: Bill Murray shows up for the 20th anniversary. (Aug. 29)

As per usual, the actor (and first-ever “Late Night” and “Late Show” guest) was there for all of the important moments — this time dressed as Liberace.

2014: Letterman announces his retirement. (April 3)

It was the tweet heard around the world from former R.E.M. bassist Mike Mills, of all people: “Dave just announced his retirement #2015,” tweeted Mills, who happened to be performing on the show that night with musical guest Joseph Arthur. Sure enough, Letterman shocked everyone by announcing that the following year would be his last in late-night TV.

His reasoning? He realized his heart just wasn’t in it anymore, particularly one day when he was so excited about figuring out a certain type of bird he spotted with his son on a fishing trip that he couldn’t even remember his guests for that night.

“If you spend most of your day trying to ID birds,” Letterman said, “should you really be running a network television program?”

2015: Letterman says goodbye. (May)

A parade of high-profile guests stopped by in the last days of Letterman’s show, which the attention-hating host treated in a typically low-key fashion, referring to his May 6 show as giving his “two-week notice.” Guests who appeared to say farewell in person included President Obama and the first lady Michelle Obama, Steve Martin, Jerry Seinfeld, Tina Fey, Martin Short, Howard Stern, Bill Clinton, George Clooney, Julia Roberts and of course, Oprah Winfrey.