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. Films produced this way usually have a well-planned pipeline, where the first film may be in post-production as the second is being shot.
While sometimes a trilogy such as The Lord of the Rings is shot with all three parts back-to-back, it is much more common for only two parts to be shot this way. 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. This approach was pioneered by the second and third parts of the Sleepaway Camp trilogy, and has since been applied to the Back to the Future trilogy, The Matrix trilogy, and the Pirates of the Caribbean trilogy. 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.