A bit of this, a lot of chat: A Cheap joke here, the chat-show in America remains a serious (and seriously rich) business. And onem man is taking it over. Giles Smith reports

This week, British viewers get to meet Larry Sanders, the American chat show host with the cheesy grin and the slick blazers. Sanders is a spoof, played by the comedian Garry Shandling, who roams the set of The Larry Sanders Show, caught up in back-stage pettiness and on-air disasters, in the most memorable of which a guest appearance by a spider-handler goes horribly wrong. The comedy is detailed, as befits an inside job: Shandling occasionally stood in for Johnny Carson on The Tonight Show, and so learned about cheesy grins and slick blazers at first hand.

In parallel, BBC 2 is screening some samples of the real American thing - The Tonight Show with its current host Jay Leno, The Late Show with David Letterman, Late Night with Conan O'Brien - so we can see where the gags are coming from, or maybe play spot the difference. In fact, the more instructive comparison might be between these bright American models and our own pale imitations.

In Britain, where there is a tradition of doing chat shows so badly that they already look like send-ups, we've just about given up on the form. Not in America. Here, an appointment such as Danny Baker's to a late-night Saturday slot earns a couple of unruffled newspaper column inches and a colour picture in the Radio Times. There, at the press conference where David Letterman announced his defection from NBC to CBS, 12 television camera crews and 100 journalists turned up. Over the next three years, if the show continues to run, CBS will pay Letterman at least dollars 42 million. Danny Baker, one imagines, comes in slightly cheaper.

MOST people assume the American devotion to late-night chat begins with Johnny Carson and The Tonight Show. And many thought it would end with his retirement in June 1992. In fact, though Carson's 30-year tenure of the show made him America's most prominent television personality, the programme pre-existed him. It was premiered on 27 September 1954, hosted by Steve Allen, a former radio presenter who has perhaps not had his due. One week before Carson left The Tonight Show, his guest was David Letterman, who told Carson, 'Thank you for my career'. Many felt, given the shape of Letterman's own show, the comment might have been better directed at Steve Allen.

Allen would break up the chat by taking cameras into the street and filming absurd skits. In one of these, he hailed a taxi, opened the door, tossed a large salami on to the backseat and shouted, 'Grand Central Station and step on it.' On another occasion, decked in teabags, he flung himself into a giant cup of water. He set the benchmark for a chat show which was about the host at least as much as it was about the guests.

You need a host of some substance to carry that off, five nights each week. Carson moved to The Tonight Show on 1 October 1962. Someone once asked him what made him a star. He replied, 'I started out in a gaseous state and then I cooled.' Letterman said of him, 'He established the model of how cool guys behaved.' Cool was the key. Carson was constantly alerting the viewer to the mechanics of the show. 'Watch this segue,' he would say; or 'I'm the prince of blends'. There was nothing (as we would have it now) 'post-modern' about this: it was simply a demonstration of impeccable assurance.

Guests liked Carson, too. Mel Brooks said, 'He listens and gets it. You can go very far out and it won't be wasted.' Carson was as witty as anyone who came on the show, but he knew when to reveal it. Other challengers on other stations came and went - Merv Griffin, Dick Cavett, Joan Rivers. Only Arsenio Hall established a significant camp lower down the mountain, and then by angling for a young vote which Carson had rarely bothered about. Carson was the last thing America would see at night before it fell asleep and, as such, America grew unusually attached to him.

When Carson retired, NBC chose Jay Leno to replace him. Leno was a former stand-up comedian who had become Carson's regular stand-in. The appointment upset Letterman who considered he had been in line for the job. Letterman is 45. At Ball State University he ran a show on the campus's classical radio station. 'That was Clair de lune,' he would say. 'You know the De Lune sisters - there was Clair and there was Mabel.' Later he got a job as a weatherman on a local television station: he was probably one of the first surrealists to take up the post. 'The hailstones today were the size of canned hams.'

For 11 years, his frequently bizarre Late Night with David Letterman show followed Carson on NBC. Non-traditional features included the 'Let's Look for Swedes' slot and 'the Monkey-cam', in which the camera was entrusted to a chimp for a while. 'It was the one real disappointment I had in my professional life,' he said after Leno's appointment. At which point his agent, Michael Ovitz, went to work to cheer him up.

Ovitz put it about that Letterman was unhappy: he probably wasn't shy about mentioning that Letterman's three million late-night audience earned NBC dollars 25-30 million per year in advertising revenue. (Tonight, an hour earlier, could charge dollars 35,000 for a 30-second commercial slot and could generate dollars 2,800,000 per week.) On two July afternoons last year, all the major players in American television visited Letterman for discussions.

Howard Stringer, the president of CBS / Broadcast Group, pitched harder than anyone else. According to Ken Auletta in the New Yorker, he got Connie Chung, the top reporter from CBS News, to send a video to Letterman in which she promised that, if he came over to the channel, for a year she would moan 'Oh Dave, Oh Dave' while making love to her husband. Possibly not for this reason alone, Letterman went with CBS. If he couldn't have The Tonight Show, he could go up against it on another channel and blitz it off the air.

The first CBS Late Show with David Letterman went out on 30 August. With a suppleness that British imitators cannot match, the show cuts both ways, offering a rich mix of glitz and left-of-centre idiocy. (Our own hosts can normally manage one, but not the other.) There's the conventional opening monologue ('Virgin Airlines have introduced gambling on their flights. Me, if I want to gamble in the air, I just fly Continental') and the usual guests touting the usual products. But recently, in the new 'Meet the Neighbours' slot, Letterman took a mobile camera into the Copy store near the studio and got the owner to photocopy her face . . .

The result is that, in the face of considerable sentimental attachment, Late Night currently out-rates Tonight by five million to four and has all but seen off the oddly nervy, deeply unfunny and critically panned Chevy Chase Show. It wasn't the route he originally planned, but Letterman has become the late-night king, in a country where that kind of royalty is still valued.

'The Larry Sanders Show', introduced by Jonathan Ross and followed by 'The Tonight Show' with Jay Leno, is on BBC 2, Friday 11.15pm.

(Photograph omitted)