There's nothing like an obituary for cutting a life down to size. When Brian Glover died last week, for example, at least one tabloid described him as "Tetley Tea star Brian Glover". "The 63-year-old actor had a string of top TV and movie credits", the report continued, "but was best known as the voice behind the hit TV ad". This combination of bathos and efficiency - "Oh yes. That one. The perforations man", you think at once - is an almost perfect instance of the "best known" construction, a bit of off- the-peg journalese without which many newspapers would be in considerable difficulties.

In the past two months this identifying two-word tag was used no less than 1,262 times in national newspapers alone - pretty conclusive evidence, I would have thought, that it has found a secure niche in the journalistic environment. And it isn't too difficult to understand why it should reproduce so successfully - for one thing it is adaptively favoured for a culture of fame, able to exploit the fact that there are now many more celebrities in the world than any sane person could possibly keep track of. Hence the need for an aide-memoire, something we can attach to all those who occupy that growing hinterland between true stardom and average obscurity.

If we extend the biological metaphor we could think of the "best known" cliche as what's called an indicator species, something like those specialised molluscs which cluster in areas where the water achieves a very specific temperature but disappear the moment it rises or falls. In a similar way "best known" is a pretty exact indicator of an intermediate degree of fame, a kind of mnemonic mussel that grows only on a certain type of celebrity. It acknowledges, for a start, that some readers may need assistance in attaching a face or a voice to the name in question. The genuinely famous won't support a "best known" - it wouldn't really make sense to say of Madonna, for instance, that she was "best known for her conical bras and lubricious stage manner" or to write of President Clinton that he was "best known for being President of the United States".

On the other hand "best known" cannot operate without some minimal consensual memory to feed off. When a star of the thirties or forties dies, someone whose public life ended long before their private one, the "best known" formula is rarely employed. "Best known for her role as Mrs Flutter in the hit radio comedy Twice Through the Mangle" wouldn't work because "best known" can't get a grip on any extant recognition. The number of people who would say "Oh yes... her" is just too small. So instead you are given a more formal list of films and plays, which you can't remember either.

As the Brian Glover example suggested, there is an unavoidable cruelty to the "best known" formula, in particular its tendency to compress a long feast of achievement into a bathetic canape. Because it is interested in recall rather than judgement it is all too liable to encapsulate a life in a rather jarring way. Jonathan Ross may like to think that he is recognised for his wit or even his sartorial elegance but the Daily Record is on hand to put him straight: "Jonathan's best known feature, of course, is his inability to roll his `Rs' ". Stephen Fry might justifiably think of himself as a writer and actor of independent status but The Times reminded him recently that he is "best known for his comic partnership with Hugh Laurie". As it happens this last instance is actually rather diplomatic in its employment of the reductive trope. Because what Stephen Fry is probably "best known" for these days is failing to turn up for a West End performance and leading the media on a wild goose chase for several weeks. But as the writer had already mentioned this in the previous paragraph he or she had to find an alternative way of filling the "best known for" box.

Occasionally writers don't quite get the point of the formula - when the Press Association reported the funeral of Vincent Hanna they wrote that he was "best known for his coverage of by-elections, his late-night Radio 5 Live chat shows and as the presenter of Channel 4's A Week in Politics". This is much more truthful as an account of the busy life just departed but it doesn't achieve the essential headline economy of the classic "best known" line.

It's true that, unlike the obituarised, Stephen Fry has plenty of time left to replace his current "best known" tag with something less diminishing. But the dread of permanence, the feeling that you may never quite shake off your existing "best known for" status, is surely one of the small anxieties of the performer's life. June Whitfield, for instance, must have been convinced that she would go to her grave as "best known for her appearance in Terry and June" before she was rescued from that fate by Jennifer Saunders. Now she appears to be "best known as Edina's mother in the BBC comedy series Absolutely Fabulous", which I think is an improvement, though she may not.

I'm not about to take up a collection here for distressed thespians - most of us will never have a "best known for" to our names and would probably quite like the idea of trying one on for size. But you can at least see how the garment might begin to pinch after a while. What would it be like to have your life summed up by something you did 20 years ago? The greater the antiquity of your "best known for", the larger the implication that it's been all downhill since then, that your life now is a mere footnote to an old achievement. It might be better, on the whole, to be best known for being yourselfn