If you're one of those old-fashioned people who goes out every morning to work and doesn't return until the evening, then it's likely you've never seen it. But for the rest of us - the unemployed, the home-makers, the freelancers, the odd-shift workers - daytime TV means one show above all. Forget Ricki Lake, Jerry Springer and their trailer-trash showdowns. Ignore the 20-minute feeding frenzy of Ready, Steady Cook. At half past four every afternoon there's only one programme worth watching. Countdown, the longest-running show on Channel 4, has a devoted audience of millions in the UK and thousands more here.
Chances are, if you catch the show for the first time when it goes primetime in its new celebrity incarnation next Thursday, you'll wonder what all the fuss is about. For Countdown has its roots in a nicer TV era, when gentle wordplay ruled the airwaves, and nobody had heard of transsexuals or tiramisu. For many people, the American sitcoms and laddish types such as Chris Evans represent the current face of Channel 4, but take a look at the most recent ratings and you'll see that Countdown beats programmes such as Frasier and TFI Friday hands down. Channel 4's viewing figures show that almost 3.5 million Britons sit down each day for a regular fix.
Currently Channel 4's fourth most popular programme (after Father Ted, Brookside and ER), Countdown is also the channel's longest-running. And although one wonders what on earth Channel 4's controllers were thinking of, the moon-faced Richard Whiteley was the first person to appear on the station's opening day all those years ago.
Whiteley - the smug, smirking schoolboy everyone loves to hate - is a crucial part of Countdown's success, as of course is Carol Vorderman, who regularly gets the male half of the pension-receiving public quivering in its easy chairs as she briskly disposes of another mathematical puzzle. At this stage, the two are like a long-married couple, their rough edges rubbed shiny by time and what looks suspiciously like mutual contempt.
But the real joy of the show lies in its simplicity - the struggle to make sense of a random jumble of letters or numbers within 30 short seconds. Despite the title, it's the word games which usually decide each contest, and the prejudice of the show is towards literacy rather than numeracy - witness the veneration for Dictionary Corner, where B-list celebrities and fresh-faced young apprentices burrow for morsels of seven letters or more.
Countdown seems such a peculiarly British institution that it comes as a surprise to learn that the format actually originated in France, where Armond Jammot's Des Chiffres Et Des Lettres has been running for more than 30 years. The British version has featured contestants aged from eight to 87 - Allan Saldanha was barely 10 years old, damn him, when he made it to the final of Series 15.
One wonders whether it's really a good idea to let the show out after teatime - until recently, it existed in its own perfectly-formed separate universe, and die-hard fans may even have had mixed feelings about Vorderman's attempts to spin off on her own with the deeply silly Mysteries With Carol Vorderman. (Of course, she also does Tomorrow's World, but that's another strange little TV ghetto.) Fortunately, Whiteley is so incredibly untelegenic that he's unlikely to be poached for any other duties.
The new "celebrity" spin-off will see the likes of Desmond Lynam, David Steel and the insufferable Jo Brand (please don't let the letters F, A and T crop up) try their hands, although the thought of Sky Sport's Andy Gray manning Dictionary Corner for The Football Show is intriguing, especially with Big Ron Atkinson as one of the contestants. In fact, most of the guests are the sort of people you'd expect to see on Countdown anyway - we're promised Bamber Gascoigne, rather than his better-known namesake Paul. Magnus Magnusson is one of the experts who will staff Dictionary Corner - in fact, the whole thing has a "Stars of the Seventies" ring to it. Obviously, despite the later slot, the old scheduling dogma that anyone watching this sort of telly must be over 65 or under five still pertains, despite all evidence to the contrary. But that's part of what makes the show such an institution, ensuring that, no matter what happens with the celebrity series, the afternoon show will go on, its glory undimmed, well into the next century.
Celebrity Countdown starts on Channel 4 on Thursday at 8 p.m.