The box office success of Jurassic Park III's opening weekend on both sides of the Atlantic shows that the public hasn't yet tired of those flesh-munching dinosaurs.

But that's the exception to the first among sequels rule that states two may be company but three's definitely one too many on the movie screen. It's accepted that most sequels are neither as good as the original nor do they sell as many tickets.

A few do manage to buck the trend, with Godfather II being the most obvious example that springs to mind. Francis Ford Coppola's sequel to his Academy Award-winning gangster epic was acknowledged to be just as good and won enough Oscars to prove it. Of course, he spoiled things but going on to make Godfather III which was an offer most cinemagoers could refuse.

Sequels to a sequel - what is the word for such a creature: a threequel perhaps? - usually stretch an idea too far, a fact enforced by the absence of many of the team that worked on the first successful one. Most film-makers grow tired of their creations before the public do.

Spielberg handed over the directorial reins of Jurassic Park III to someone else, just as he abandoned Jaws after the original shark shocker had audiences jumping out of their seats. Releasing the third in 3-D, as in Jaws 3-D, was an attempt to inject new life into a tired idea. Omitting the numeral after the title and calling the fourth Jaws: The Revenge didn't fool anyone into believing that the franchise was anything but past its sell-by date.

Studios like sequels because they save thinking up new ideas. They can recycle a successful formula more quickley and cheaply than spending ages in development hell thinking up new ideas. Some sequels like to call themselves series, for added respectability.

For those like Batman, selling the merchandise associated with the project can be more profitable than the money made on the movie itself.

The Bond films, Planet Of The Apes, Rocky, the Carry On comedies and Clint Eastwood's Dirty Harry thrillers have all proved to have long-running potential.

Performers too can benefit financially. Space chiller Alien made Sigourney Weaver a star as she battled unfriendly monsters. She carried on fighting in Aliens and Alien 3, and was eventually persuaded to don space suit for a fourth time in Alien Resurrection for a bumper pay packet. Her character, Ripley, had been killed off in number three but the actress and series had become so closely associated that the producers were hesitant about doing another without Weaver. Less successfully, Charles Bronson continued seeking vengeance in the Death Wish thrillers (five in all) long after the departure of original director Michael Winner.

Makers of horror movies feel almost obliged to continue ad nauseam. The low budget, gory Friday The 13th was a surprise hit, so a sequel was inevitable. Few would have imagined it would spawn eight sequels. Jason Goes To Hell: The Final Friday was the last although there has been talk that Jason would be revived yet again to do battle with Freddie Krueger, whose razor-fingered antics terrorised audiences in five Nightmare On Elm Street shockers.

That's the trouble with movie villains - when they're dead, they won't lie down. Maniac Michael Meyers stabbed his way through no less than six Hallowe'en movies, then was given a new lease of life in Halloween: H20 when original scream queen Jamie Lee Curtis reprised her role one more time.

Three seems to be the sticking point for many non-horror film sequels. Spielberg did stay around long enough to direct all three with Harrison Ford as the whip-cracking archaeologist, but rumours of a fourth have failed to materialise into something solid.

The same team also stayed together for all three Back To The Future adventures, although the second and third were actually made back-to-back and released several years apart. This saves time and money but represents a bit of a gamble. If audiences stay away from the sequel, then the third episode might just as well be binned.

This method of sequel-making is popular at present. Cameras start rolling on two sequels to The Matrix in Australia at the end of the summer. This Christmas sees the debut of the $270m film version of Tolkien's Lord Of The Rings. Director Peter Jackson has been toiling away for years in his native New Zealand on the eager-awaited screen adaptation. One reason it's taken so long is that he's making all three films in the trilogy in one go. He's confident that the story is strong enough to make cinemagoers eager to see the second and third episodes. "We want the moviegoer to know that it's a trilogy and that the quest to destroy the ring will happen at the end of the third movie," he says.

George Lucas tried a different method with Star Wars. Making three - the original Star Wars, The Empire Strikes Back and Return Of The Jedi - and labelling them episodes four, five and six. Then, after 15 years, he embarked on making the first three episodes, starting with The Phantom Menace.

With this year's summer release schedule reading more like the football results than ever before - Jurassic Park III, Dr Dolittle 2, Scary Movie 2, Pokemon 3, Rush Hour 2, Crocodile Dundee 3 - there's no sign that sequel-mania is abating.