Soap opera and sitcom are the two forms of television drama which are most clearly rooted in the early history of the medium; for that reason some critics argue that they are the "purest" forms of television, with very different conventions from, say, filmed serial drama like Ballykissangel, ER or Prime Suspect.

Soap's roots in the 1950s are still obvious from these dramatic rules, which derive from the technology of the time, when nearly all programmes were broadcast live. The introduction of videotape in the early 1960s allowed programme-makers to record and edit their programmes in advance, but many of the stylistic conventions of early television remain to this day in soaps like Coronation Street, EastEnders and Fair City, which are shot by multiple video cameras and edited as they are being filmed.

Soaps nowadays are recorded weeks in advance, on a complicated schedule, but each episode still takes place "now", almost always over the course of one fictional day; only very rarely will a programme run over into a second day.

Equally, in Coronation Street you will never see a jump cut, cutting directly from one scene featuring a character to another scene with the same character in a different location; this is a common device in cinema or filmed TV drama, but not in soap.

This convention helps to ensure that the illusion of real time is maintained, but it also has its roots in early live television, when actors needed time to change their clothes, or to move from one set to another. (Last week in the US, NBC launched its new series of ER with a much-hyped episode that was broadcast live, in the style of those old TV shows).

Soaps rarely, if ever, reflect what is happening in the outside world - political events, natural disasters and even changes in the weather generally pass them by. This is partly due to the demands of the production cycle - most soaps are recorded several weeks before transmission, and it's not possible to react quickly to events. But soaps also steer clear of current events because they would be an unwelcome distraction into their self-contained worlds.

In the week after Diana's death, only Brookside referred to the emotional outpouring going on around Britain, slipping a hastily recorded conversation about the subject into Friday's episode. The effect, ironically, was highly artificial - the scene looked tacked on - demonstrating that soap rules are broken at the producers' peril.