When the BBC's drama department decided to show viewers what was happening in the wards that formed the backdrop to Casualty, its long-running medical drama Holby City was created.
Seven years on, BBC1 is now planning a spin-off of a spin-off - a drama set in a police station close to the fictional hospital.
It is the latest in a long line of programmes to have spawned new shows, both in drama and comedy, with varying degrees of success, from the hugely popular Cheers spin-off, Frasier, to the less successful Friends follow-up, Joey.
Provisionally titled Holby Blue, the series is co-produced by Tony Jordan - the veteran EastEnders writer who created the recent BBC1 hit Life on Mars and the slick con artist drama Hustle.
His company, Red Planet, is in the early stages of making the pre-watershed show, which is likely to fill the poorly performing Wednesday night slot that BBC1 controllers have struggled to fill. A creative team is currently deciding how much crossover there should be between the parent show and its offspring. A BBC spokeswoman said: "We're in really early stages of script development. How and why it will be linked to Holby, we're just discussing. Holby is a key part of a fantastic brand that does really well for us."
Holby City was launched in 1999 by its drama chief at the time, Mal Young, who had been unimpressed by blueprints for other hospital dramas. He decided instead to build on the strengths of Casualty, which had been running for 13 years.
Apart from the occasional Christmas special, characters from Holby City and Casualty rarely appear in the other show.
The faster-paced Holby City, which draws an average eight million viewers to its pre-watershed Tuesday night slot following EastEnders, attracts a younger audience. Its glamorous cast includes Tina Hobley, Hugh Quarshie and the Footballers' Wives actor Ben Richards, while previous stars have included Angela Griffin, Lisa Faulkner and Jeremy Edwards.
Steven Murphy, the editor of Inside Soap, said: "From a BBC point of view, it's a much safer way to launch a new show than doing something cold. Holby gets eight million viewers and you hopefully bring a proportion of those with you. The worry is you end up with five hours in one week which are very much of one style, in the same genre, with the same stories ... it's all going to get a bit samey."
Details of the new show emerged just days after BBC director general Mark Thompson announced a new focus on long-running dramas with broad appeal. Mr Thompson told staff: "We must cherish and where necessary invest in the titles that bind us emotionally to the broadest audiences - EastEnders, Casualty, Holby."
ITV successfully attempted something similar in 2003 when it launched The Royal, set in a cottage hospital near the fictional North York Moors village of Aidensfeld, the location for its popular Sunday night police drama Heartbeat .
Frasier was one of the most successful spin-offs in television history. Launched when the Boston bar comedy Cheers drew to a close in 1993, it followed one of the regular characters, Dr Frasier Crane, as he returned to his home town of Seattle, where he hosted a radio therapy show. Joey, starring Matt LeBlanc, reprised his character from Friends, but failed to repeat the success of the New York sitcom and was pulled halfway through its second run.
Ratings winners and losers
SPIN-OFFS THAT WORKED
When Cheers came to an end, one of its main characters, Dr Frasier Crane, moved from Boston to his home town of Seattle, where he lived with his father and worked as a radio psychiatrist. Audiences were so charmed that the sitcom ran for 11 years, the same length of time as the show that spawned it.
In 2003, ITV launched a spin-off to its popular Sunday night police drama Heartbeat. The Royal, set in a nearby cottage hospital, shared a soundtrack of Sixties classics, but characters and storylines were quickly differentiated, adding to the show's success.
The Green Green Grass/Only Fools and Horses
When Del Boy's creator John Sullivan decided to relocate the second-hand car dealer Boycie and his wife, Marlene, to rural Shropshire, 9 million viewers tuned in to see the result. The BBC liked it so much they commissioned a Christmas special and a second series.
Softly Softly/Z Cars
Z Cars, launched in 1962, featured police officers on the beat in fictional Newtown, near Liverpool; Softly, Softly focused on CID officers in the same force. It ran from 1966 to 1976.
Kevin Whately was reluctant to reprise his role as Inspector Lewis after the death of his co-star John Thaw. When ITV finally persuaded him to go back to Oxford for a one-off pilot, they liked the result so much they have now commissioned four more episodes.
... AND THOSE THAT DIDN'T
Friends was always going to be a hard act to follow. The spin-off starring Matt LeBlanc initially gained respectable viewing figures but NBC pulled it halfway through its second season after ratings slumped.
The sequel to Porridge, broadcast a year after the original sitcom ended, followed the exploits of Norman Stanley Fletcher after his release from Slade Prison. It failed to capture viewers' imagination and has rarely been repeated.
Swiss Toni/The Fast Show
Flagged as one of the highlights of the new digital channel BBC3, there were high expectations for Swiss Toni , a comedy based on the character from The Fast Show. But audiences seem to have tired of his habit of comparing every activity to "making love to a beautiful woman".Reuse content