Thomas Sutcliffe: The question every scheduler asks when writers knock at the door

How easy it is to think of the watershed as being an immemorial feature of broadcasting geology, a regulatory rock of ages.

In Broadcasting Standards Commission documents it is actually given a capital letter, as if it is a God, rather than an informal contract between broadcaster and parent. Then there's the image itself, one that conjures up a granite ridge separating the world of childish innocence from a broad valley of adult material. For most television professionals working now, there's never been a time when it wasn't there – a natural obstacle you simply can't ignore.

In truth, it's little more than 40 years old. In 1958, the influential Himmelweit Report on Television and the Child prompted much debate on the sensitivities of young viewers. The O'Connor committee filed its report on the matter in 1960. By 1962, after a largely informal process, the notion of the watershed was firmly in place.

Before then, there hadn't been much need. British television was aimed principally at a family audience and was so avuncular in its attitudes it even incorporated an hour's suspension of broadcasting between 6pm and 7pm to allow smaller children to be put to bed. The Toddler's Truce, as it was known, ran from the resumption of broadcasting in 1945 to 1957, until the new ITV companies protested that it represented too much of a financial loss for them.

However, as television began to flex its creative muscles in the Sixties – and to respond to the changing social attitudes – some kind of cordon sanitaire was clearly necessary. Now, the watershed dominates the scheduler's sense of the evening landscape. "There will always be a moment when I'm having a conversation with a writer and I'll say 'Is it pre or is it post?'" explains Jane Root, the controller of BBC2.

Not many programmes will work effectively either side but they are, Root acknowledges, "a fantastically useful scheduling weapon". And even though broadcasters recognise that domestic habits have changed enormously since the watershed was first introduced, the cut-off remains a useful demarcation as to when parental vigilance should increase.

Even children recognise its value, apparently. Dr Maire Messenger Davies, a senior lecturer at Cardiff University is about to publish Dear BBC, a study of the regulation of children's television. As part of her research, she invited children to build their own evening television schedules and found that almost all of them observed the watershed – usually on the basis that their younger siblings needed protection.

The children were also critical of cable and satellite channels, which were seen as infringing the concept of the watershed, though Dr Messenger Davies wisely notes that this may at least in part be owing to the perceived romance of what lies unseen just beyond that granite ridgeline.