Sometimes, two or more films in a series are shot and produced "back-to-back", which means simultaneously or within a short space of time.
This is usually done to eliminate the need to rebuild sets and re-hire actors for sequels, and maintain audience interest in the film series.
Often, in a trilogy, the first film will be made on its own, and if it is a success, the remaining two parts will be produced back-to-back.
Back to the Future Part II, and later, The Matrix Reloaded both ended with the words "To be concluded," a variant on the traditional "To be continued," and a trailer for their respective upcoming sequels.
The following is a list of films that have been produced this way: Though not shot entirely back-to-back, the final scene of Star Wars: Episode III – Revenge of the Sith was shot in Tunisia during the production of Star Wars: Episode II – Attack of the Clones in order to avoid another trip to Tunisia for a single scene.