Movie Multipack

Making multiple films together. Normally Hollywood waits until it knows it's got a hit before ordering up a sequel, but that approach has problems. If they're lucky, the original writers will have left Sequel Hooks, but the seams will still be visible, and they've got to get the original cast back together.

It's so much simpler to make the sequel before the first film is released. The stories can be written to fit together smoothly, and none of the cast are going to disappear, or demand more money.

Movie multipacks come in three varieties.

Two sequels for the price of one. Following a hit film with a two-pack of sequels, to complete the trilogy. Examples:
 * Back to the Future
 * The Matrix
 * Pirates of the Caribbean
 * Star Wars

One story in N parts. When the story is too long to fit in just one part it can be split over several films, all but the last typically ending in a Cliff Hanger. Examples:
 * The Three Musketeers (The Richard Lester version from 1973) was shot as one film but released as two. Lester got sued by some of the cast, who had only been paid for one film and felt they should have been paid for two.
 * Kill Bill
 * Superman; written as a 5-hour epic, shot as two movies, though the second was mostly reshot after Richard Donner was fired.
 * Death Note
 * The Lord of the Rings

String of stories. Particularly with book adaptations, the story may naturally come as a multi-volume epic. Each individual film has closure, not a cliffhanger, but together they form a greater whole. Examples:
 * Harry Potter
 * The Chronicles of Narnia

All of these varieties are often sold as Boxed Sets.