On many occasions I have told people that early on in my life I came to the realization that my ideas about what was and wasn’t funny were shaped by three men: my father, Bill Murray, and David Letterman.

My dad had a dry intellectual wit and remarkable dexterity with the one liner, Murray has always displayed a rascally comedic derring do, and for over thirty three years–longer than anyone in the late night television universe including his mentor and idol, Johnny Carson–Letterman had combined both with his own idiosyncratic genius to change the format forever. On May 20th, that will end.

It is truly impossible to underestimate the ways that Letterman, as irreverent an icon as broadcast television has ever known, changed the talk-show playbook. Prior to him they were just that, shows on which people talked. From the roots of his ill-fated morning program, The David Letterman Show, it was clear that this was a host interested in pushing boundaries. Often quite literally as when he pushed beyond the established, and often sterile, constraints of the studio and went off into the world outside looking for adventure.

It now seems hard to imagine, but prior to Letterman, television hosts simply didn’t do that. They ruled their controlled environments, chatting with guests who had something to promote and providing a little musical entertainment. Letterman blew that model up. He would welcome, and occasionally spar with, celebrity guests but he was just as likely to get laughs as he worked the McDonald’s drive-thru in suburban New Jersey or tried unsuccessfully to deliver flowers to General Electric after they purchased his then employer, NBC.

From the start Letterman seemed intent on combining the traditional talk show format dating back to Jack Paar and Steve Allen with a more inane sense of possibility perhaps best seen previously in the variety show leanings of Ed Sullivan. Letterman is a man who allowed your human neighbors or their pets to appear on national television to show off their stupid tricks, who wore a suit made of Alka Seltzer into a tub, and who made on-air talent out of neighbors to his Ed Sullivan Theatre and his show staff while also interviewing presidents and key policy figures. As he once opined, “The road to the White House runs through me,” and no candidate other than John McCain seemed prepared to call him on that (and McCain, after skipping the show on the pretense that he was suspending his campaign only to be seen in makeup preparing for the CBS Evening News, came back hat in hand, admitting, “I screwed up”).

Letterman has provided his successors a much larger palette than the one he inherited but as the new breed of late night hosts, we’ll call them “The Jimmy’s,” have taken to skits and games with focus on the value of the viral they have lost what may have made Letterman so monumental, and that was his edge.

Dial up any appearance by Bill O’Reilly and you will see Letterman pull no punches. His initial interactions with both Madonna and Cher are priceless and it’s worth noting that both went from openly regarding him as an asshole to being smitten with his smart-ass charm. As Tina Fey, who was among the incredible list of celebs visiting the show in the last month, said in the recent Rolling Stone cover story on Letterman, “Once Dave retires there’s nobody left to be scared of. It’s all friendliness now.” Watching Jimmy Fallon respond to every utterance from a guest like an excited puppy does not stand on the same level as watching Letterman call out a chump. It seems unfathomable that Jimmy Kimmel will one day have the gravitas to host a television program appropriately after an event with the historic significance of 9-11, or the balls to do one without an audience as Letterman did after Hurricane Sandy. Kimmel is, however, running an “out of respect” repeat on the night of the final Late Show with David Letterman, so perhaps there’s hope.

I have been asking people regularly if they have been tuning into this remarkable last month of the show, emphasizing that this is truly historic television. One of the few things in the world that an American television viewer could count on, night in and night out, after a good day or a lousy one, was that David Letterman was going to be there at 11:30. In thinking about those three men that shaped my comedic sensibilities it has always been Letterman in that time slot and on May 20th that ends. My dad passed away earlier this year, so that leaves me with just Bill Murray now (the first guest on all of Letterman’s late night ventures and a good bet to be his last). I’ll be counting on him, and I know that I’m not the only one.