Origins: it began life in 1983 as a five-minute feature on TV-am. Kevin Sim, the breakfast channel's features editor, originally contacted Loyd Grossman - then design editor at Harpers & Queen - by mistake, having 'confused him with another journalist with an unusual name' (no one will ever admit who this was). David Frost's intellectual weight then helped carry the concept to Yorkshire TV and the grandeur of a full half-hour.
Frequency: one series a year, of between 11 and 15 episodes. The traditional Friday-evening slot has now been swapped for Sunday - offering fans of Grossman a tempting double bill with Masterchef.
Ratings: steady. Between 7 and 8 1/2 m.
Formula: Frost introduces three panellists - Willie Rushton, Eve Pollard and Andrew Neil would be a typical trio. He then hands over to Grossman, who strolls around the meticulously tidied house of an unknown celebrity, making observations on their decor and speculating about their character, before coming out with the immortal catchphrase 'Who lives in a house like this?'. Back in the studio, Frost gives the viewers at home and the studio audience (but not the panel) a quick peek at the mystery host, who is sitting behind a screen in a state of considerable psychological discomfort. The panel then take it in turns to make jokes about the house and insult Loyd, while endeavouring to guess the occupant. When the panel succeeds, or their failure has become embarrassing, he or she emerges to talk about his or her latest book/series/manifesto/athletic achievement and be presented with a Through the Keyhole key - a memento which challenges the Crackerjack pencil and the Blankety Blank chequebook for desirability. After a commercial break, the process is repeated for a second guest.
Who appears on a show like this? Robert Maxwell, Bernard Manning, Paddy Ashdown, Cecil Parkinson, David Icke, Cleo Laine and Johnny Dankworth, John Prescott and Robin Cook, but not Tony Blair. Among many imaginative pairings, Mary Whitehouse and Five Star stand out.
Why do they do it? The obvious answer is money. Guests get paid just over pounds 1,000 for having their nooks and crannies scrutinised and coming to the studio to risk the ultimate humiliation of 'beating the panel' (ie not being famous enough to be guessed). The producer, Chantal Rutherford-Brown, admits with engaging candour that it's not just the cash that draws them in. 'We get more and more people approaching us, because there are very few shows now where a celebrity with something to promote can reach such a large audience. We're not averse to them doing that at all, because it makes the show topical.'
What is the Through the Keyhole combined theory of celebrity inversion? The least prepossessing panellists - Derek Nimmo, Eamonn Holmes of GMTV - often make the best guessers; and the tackiest guests - John McCririck, Screaming Lord Sutch - often have the most tasteful houses.
Anything that makes you want to kick the set in? The way the camera keeps cutting back to the celebrity's face for his or her reaction to the panel's comments, making it impossible to play the game along with the experts. More seriously, the fact that it is still felt necessary to telegraph the presence of a black guest by emphasising such dubious signifiers as ' 'colourful' decoration and fondness for reggae'.
The bottom line - how good is it? Through the Keyhole might have been devised by Lord Reith to illustrate his three founding principles of quality television. It educates (if Garry Bushell asks you to house-sit, for heaven's sake say no), informs (athletes get paid a lot more than you might think) and above all entertains (gasp at David 'Diddy' Hamilton's toilet decor, chuckle as David Frost introduces Chris Tarrant as 'the Blond Bombsite'). The recent innovation of including surreal extra clues - a large portrait of Allan Lamb on his hall wall, a Suzi Quatro tour-poster in Russian - has, if possible, made the show even more enjoyable.