Copy editors are ruthless. Their function is to make sure that the text is prepared for the printer. They are not literary critics, and must plough through any amount of sentimental rubbish without wincing, turning a blind eye to cliche, weak characterisation and barking-mad improbability, provided the chronology works and the heroine’s hair doesn’t keep changing colour.

But there is other territory, too, a literary no man’s land where correctness is a matter of taste. Roaming this, swishing their pencils like Malacca canes, they scorn my use of commas and deadhead my flowering semi-colons. They draw wiggly lines under repetitions, sometimes intentional ones. They beg to differ about the way people talk: you may have heard your characters speaking in your head, but if the copy editor prefers all characters to speak in correct grammar, or doesn’t accept that “yeh” correctly renders a particular kind of “yes” - in goes her version and you furiously rub it out. They are ruthless about metaphors, too. “Can an office be described as “worried?” they ask, with barely veiled contempt.

But just as you are about to drive round and kill them with your bare hands they save your bacon by writing some utterly necessary message in the margin like “the baby was only four months old on page 154, how come it is sitting up?” or “he can’t have worked there seven years, he’s only 27 and he went to Harvard”.

We need them, but some copy editors have such an idiosyncratic approach that it is unwise for the sensitive author to look at their strictures without a large india rubber in the fist.

Such people - indispensable to quality and dangerous to self-esteem - are at their most infuriating when they are right.

That is why they waft you back in time to your school years. Sometimes, coming to the end of a laboriously edited chapter, I have found myself looking round in a puzzled fashion for the mark - 810, surely? Or an alpha minus? Or maybe a sarcastic little comment such as “you would do better if you spent more time with the sources before you start” or “a little too fond of your own theories, Elizabeth”.

The nearest I get is my friend Tina, who when asked to scan a typescript writes furious remarks like “we know this already, we are not stupid!”

Children care about their work as much as adults do. As a parent, I have always been fascinated by the different approaches teachers take to writing all over people’s work with angry red pens. Comparing notes with friends, it would seem that in the primary sector there was a long period (ending in the mid-nineties) when it was unfashionable to red-pen every spelling mistake. “It’s all smiley faces,” grumbled one parent, who kept his own green pen to do the corrective job.

Only twice have I encountered the superliterate parent’s nightmare: where teacher has flamboyantly corrected the child’s spelling when the child was right in the first place. A friend had this happen so often to her bright seven-year-old that she remonstrated; the frost lasted all the way to Year 6.

More interesting is the text that sixth- form teachers deliver in the margin of essays in history and English. There can be a real buzz of intellectual excitement expressed in the small, intensely tidy writing:

“Charles the Bold said something very similar, I can lend you the book” or “I don’t know how you can say he was upper-class, his father was a station master” or “Garibaldi would have known...”

Sometimes, as with a sympathetic fiction editor, marginalia are good company, part of shared enjoyment. Sometimes, they are unsympathetic or almost malevolent. But you’d miss them if they weren’t there.

I know a writer of argumentative history books who hauls his own works out in university libraries in the hope of finding marginal notes. He once found “wrong, wrong, wrong, you stupid git”, but usually has to content himself with underlinings, exclamation marks and queries.

He has yet to share the experience related by Alan Bennett, who found a passage in a book marked by a long, wavering pencil line in the margin. He read it several times very attentively to try and see what had so moved the previous reader. After a while, the line blew away: it was a hair.

Libby Purves’ latest novel, properly corrected last year, is “A Free Woman” (Hodder)