For two decades, Thursday nights belonged to NBC with its formula of four comedies and the best drama the network had to offer. It netted both big audiences and large amounts of advertiser dollars that made the competition green with envy. Network publicists even coined a phrase for its juggernaut: “Must See TV.”

But those days are over.

When the networks announced their respective fall schedules in May, NBC told advertisers it would schedule on Thursdays at 9:00 a new comedy drama from “The West Wing’s” creator Aaron Sorkin, called “Studio 60 On The Sunset Strip”–a look at a fictional network late-night sketch show (not unlike NBC’s “Saturday Night Live”). Just two days after NBC’s announcement, ABC unveiled ITS fall schedule, which included the move of its top-ten medical drama “Grey’s Anatomy” to do battle on Thursdays at 9:00–against CBS’ dominant police drama “CSI” and “Studio 60.” Suddenly, critics began speculating when (not if) NBC would move “Studio 60" to another night. The answer came a week later, when NBC Entertainment chief Kevin Reilly announced several fall time shifts: “Studio 60" would air on Mondays, while the midseason game show hit “Deal or No Deal” would do battle with “Grey’s” and “CSI.” If there was a metaphor for NBC’s sinking fortunes on a night it once controlled, this was it.

The seeds of “Must See TV” actually took root in 1982. In those days, NBC was third in a three-network race; there was no Fox, UPN or WB; and cable was a growing force but not yet the competitor it would eventually become. CBS was the network to watch on Thursdays, with “Magnum, PI;” “Simon & Simon” and “Knots Landing.” All three landed in television’s top 20 and helped keep the Eye Network on top.

Not that NBC didn’t try to break the CBS stronghold. For the fall of 1982, NBC President Grant Tinker and head programmer Brandon Tartikoff created what promos called “the best night of television on television.” At 8:00 PM there was “Fame,” the well-crafted drama based on the film about a performing arts school and its students. At 9:00, Tinker and Tartikoff rolled the dice by introducing a new “gang comedy” about a Boston bar called “Cheers,” followed by the Emmy-winning sitcom “Taxi,” which was picked up by NBC after ABC cancelled the show. “Hill Street Blues,” NBC’s much lauded police drama that began building an audience the year before, rounded out the evening.

The results for the 1982-83 season were mixed: Only “Hill Street” landed in the top 25; “Fame” didn’t do much business and moved to syndication the following season; while “Taxi’s” audience numbers plunged and the show ended its run for good. But “Cheers”–which started at the bottom of the ratings–slowly built an audience and was renewed for a second season.

The fall of 1983 saw a revised NBC lineup of four comedies plus a drama. Returning shows “Gimme A Break” and “Mama’s Family” aired from 8:00 to 9:00; followed by a new sophomoric sex comedy called “We Got It Made” at 9:00. Wrapping up the night was “Cheers” and “Hill Street”. “Made” lasted only one season and “Break” was moved to another night. “Mama’s Family” went to syndication and was replaced in midseason by the comedy “Family Ties” with Michael J. Fox (which was doing so-so on Wednesdays)..

But it was the Fall 1984 lineup that finally did the trick for NBC. The network led off with a new family sitcom starring Bill Cosby–who had failed with his previous two variety series but stayed in the public eye through commercials. “The Cosby Show” was different. It was a well-produced and executed domestic comedy about parents who really did know best, and their children who made mistakes but were otherwise decent kids. It was Cosby at his best and audiences didn’t take long to embrace him. “The Cosby Show” quickly became television’s most-watched program and viewers kept their dials on NBC.

“Cosby” propelled the show that followed it, “Family Ties,” to second place. “Cheers” and the more slapstick adult comedy “Night Court” also benefited, as did “Hill Street Blues.” That performance pulled NBC into second place for the first time since 1975.

By 1986, all four NBC Thursday sitcoms landed in the top ten, led by “Cosby.” They helped NBC become the number one network–a position it last held in the early 1950's. The Thursday schedule remained in place until the spring of 1987, when Steven Bochco’s “Hill Street Blues” was moved to another night as it ended its seven-year run. Its replacement was another Bochco-produced series–the legal drama “L.A. Law”–which landed among television’s top 20.

Bill Cosby’s clout was evident in the fall of 1987, when “Family Ties” moved to another night and was replaced with a “Cosby” spinoff called “A Different World.” Everything else stayed the same but the ratings, with NBC continuing to dominate.

But on April 30th, 1992, “The Cosby Show” aired its last original episode. That fall, NBC faltered in the ratings; only “Cheers” (in its final season) was a top-ten hit for the network. But in early 1993, NBC gambled by slotting the low-rated but critically acclaimed “Seinfeld.” behind “Cheers.” The “show about nothing” soon landed in the top ten. By the fall of 1993, “Seinfeld” was the new Thursday night anchor, helped by the very successful “Cheers” spinoff “Frasier.”

The fall of 1994 cemented NBC’s place in the history books. “Frasier” was moved to shore up Tuesday nights (a move that proved to be a smart decision); its replacement, “Madman of the People” stuck around for just one year. But a saga about six single people living in New York City became a monster hit, as did “L.A. Law’s” replacement--a new medical drama set in a fictional Chicago emergency ward.

“Friends” and “ER” were immediate top ten smashes, giving NBC three strong anchors on what was now the most powerful night on television. And unlike the “Cosby” days, the new generation of NBC Thursday comedies focused on single people, a trend other networks picked up on and copied.

As the 1990's rolled on, the NBC Thursday juggernaut kept smashing records and generating millions of dollars in advertising revenue. With relatively young male and female viewers watching the gang at Central Perk, observing the newest struggles at Seinfeld’s New York apartment or watching the latest case roll through the emergency room at Cook County General, advertisers were putting their dollars into NBC. Movie studios began promoting their latest releases to NBC’s audience of 40 million viewers every Thursday night. The other networks were reeling.

CBS began airing news programs and dramas aimed at older audiences; ABC had the news magazine “Prime Time Live” but not much else; Fox began airing shows aimed at African-Americans who would not watch the mostly white lineup on NBC (“ER” was the notable exception); WB aimed its schedule at young women while UPN aired wrestling and grabbed about three million young men a week. By this time, NBC was promoting its lineup with the “Must See TV” tagline, the creation of an unknown network executive and airing on endless promo after promo, even spreading to other shows and/or nights that NBC saw fit.

But NBC’s Thursday juggernaut began showing signs of vulnerability. After all, there was only so much “must see television” that could be created. As a result, NBC had three strong anchors (“Friends,” “Seinfeld” and “ER”), propping up one second-rate sitcom after another (“The Single Guy;” “Caroline In The City;” “Jessie;” “Suddenly Susan”). Those shows were either cancelled or moved to other nights because they lost a significant share of the audience from “Friends” or “Seinfeld.” Critics called those programs “walk the dog shows” because viewers tuned out, took care of other business, then returned to NBC for the good stuff.

The search for successors to the “Must See” classics accelerated in late 1997, when Jerry Seinfeld told NBC executives he wanted to end his show while he was still proud of it. Even when offered five million dollars an episode, Seinfeld was adamant: His show ended for good in May 1998.

But NBC didn’t find a new “Seinfeld” successor in the fall of 1998. It ended up taking “Frasier” (which had rebuilt NBC’s Tuesday lineup) and moving it to Thursdays as “Seinfeld’s” replacement. “Frasier” did well enough, but the show was now in its fifth season while “Friends” and “ER” were four years old. To keep its lucrative Thursday night intact, NBC began throwing money at the studios and cast members to keep all three shows on the schedule. It became a never-ending cycle: The more mediocre sitcoms NBC aired and pulled from the schedule, the more the audiences tuned out for them, and the more the network was forced to rely on its aging hits. NBC was still master of its Thursday domain, but the margin of victory was getting smaller every season.

In the summer of 2000, CBS surprised everyone (including network executives themselves) with the unexpected smash that was “Survivor.” With more than 50 million people tuning in for the first season finale, CBS quickly renewed the reality show for a second season, with plans for a January 2001 premiere. CBS received another major break that fall when a new crime drama became a surprise hit on Fridays. “CSI” was chalking up surprisingly high ratings with a lack of promotion. With two hits in its arsenal, CBS Entertainment chief Les Moonves–who ironically sold “Friends” and “ER” to NBC while heading Warner Brothers’ television division–went for the kill and announced both “Survivor” and “CSI” would move to Thursdays in January.

NBC’s new head of entertainment Jeff Zucker (a former producer of the network’s lucrative morning “Today” program) knew he was in trouble. The network’s latest post-“Friends” comedy called “Cursed” (a.k.a. “The Weber Show” after its star Steven Weber) turned out to be another loser. NBC had moved “Frasier” back to Tuesdays, and moved in “Will & Grace,” its strongest anchor comedy in some time–followed by the so-so gang comedy “Just Shoot Me.” Meanwhile, strongest actors (George Clooney among them) began leaving the show, and its ratings were starting to slip.

Zucker’s answer: “Supersize.” In other words, stretch “Friends” and “Will” to 40 minutes each and insert a high-profile show in between (a “Friends” clip show; live sketches from the “Saturday Night Live” cast”) to blunt “Survivor” and “CSI”–while dumping the mediocre “Cursed” and “Just Shoot Me” during the crucial February 2001 sweeps. CBS’ schedule moves worked, but the NBC stunting allowed the network to win Thursday nights, especially among the young audiences NBC needed to draw advertisers–at least temporarily. Zucker also used guest stars to bring in audiences for “Friends” and “Will & Grace”–sometimes successfully, other times without making a dent in the competition. CBS’ lineup continued to rise.

By the fall of 2003, “Survivor” battled “Friends” for first place;” “CSI” was television’s top-rated drama; and a new CBS crime drama called “Without A Trace” started beating “ER” in head-to-head competition. CBS now won Thursday in total viewers (but not the 18 to 49 viewers NBC courted.) Even worse, the latest “must see” comedy offering from NBC–an Americanised version of the UK hit “Coupling”–was quickly rejected by viewers after a few episodes. Add the fact that it was the last season for “Friends” (and “Fraiser”), and NBC was at a crossroads. But in January 2004, NBC juggled its lineup by moving “Will & Grace” a half-hour earlier; killing off “Coupling” and another bomb called “Inside Schwartz,” and slipping its budding hit reality show “The Apprentice” with the egocentric New York real estate mogul Donald Trump to do battle against “CSI.”

The strategy worked; “The Apprentice” pulled nearly even with “CSI” in audiences that season while beating it among in younger viewers. But it also signalled the end of NBC’s Thursday legacy of four comedies and a drama, something the industry was quick to notice.

For the fall of 2004, it was good-bye “Friends,” hello “Joey.” NBC decided to spin off Matt Le Blanc’s Joey Tribbiani and move him from New York to Hollywood. It was a strategy that worked with “Frasier.” But Kelsey Grammer’s Frasier Crane was a more rounded character (as was the actor who played him) and producers made sure to create a show that in style and tone was a complete departure from “Cheers.” By contrast, Joey Tribbiani felt more like a second-banana in his own series, and supporting cast members were either dropped or recast. Audiences initially followed “Joey” when it premiered, but the ratings soon dropped off to well below the numbers once achieved by “Friends.” “Will & Grace” also felt the lack of audience support and its own aging format; “The Apprentice” was starting to show signs of vulnerability in its second go-around; and “ER” fell out of the top ten for the first time in its history. CBS was the new Thursday night champ in both total viewers and those between 18 and 48. NBC–which was number one the season before–fell to fourth place. That was the steepest drop of any network in modern television history. Worse, it lost one billion dollars in advertising revenue for the upcoming season, thanks in part to the Thursday free-fall.

When the fall 2005 schedule was announced, advertisers and competitors were shocked. The NBC Thursday lineup remained exactly the same, with “Joey” getting a second season and “Will & Grace” entering what would be its last year. “The Apprentice” was no longer a “water cooler” reality hit (a short-lived version with homemaker/businesswoman Martha Stewart diluted the Donald Trump edition, as did Trump’s public ego). “ER’s” free fall continued. The fall 2005 season was a disaster. In early 2006, NBC shoved “The Apprentice” off Thursday nights and put “Joey” on hiatus; it would eventually be cancelled. In place of “The Apprentice,” NBC substituted its biggest new fall series, the comedy “My Name Is Earl” and the increasingly popular US version of “The Office.” Ratings improved, but NBC was still a poor second to CBS, while ABC found a surprise reality hit with “Dancing With The Stars,” which easily beat the NBC lineup.

No matter what happens in the fall (or the years to come), it’s clear the NBC Thursday legacy won’t be duplicated anytime soon. (I don’t see myself setting the hard drive recorder to watch “Deal or No Deal.” But then, one day, CBS will have to search for replacements for “Survivor” and “CSI.”)

At its best, “Must See TV” gave Americans some of the best comedy and most thoughtful dramas in broadcast history. It raised the bar for good shows–and even though NBC has squandered its heritage, we TV fans owe the network a debt of gratitude for giving us two decades of uplifting, intelligent entertainment. And knowing the Peacock, you’d be a fool to rule out “Must See TV: 2010!”