The continuing use of U.S. customary units has caused writers to speculate in fiction set in the future about the adoption of the metric system. Many authors have assumed that the United States and the human race in general will use metric units; however, others have assumed the continued use of U.S. customary units or have neglected to take metrication into account. Writers sometimes use customary units simply because American readers and watchers will understand the measurements; other times, they are simply used by accident. The 1966 show Star Trek, for instance, initially used U.S. customary units despite multiple Earth national origins and a setting in the 23rd century. Later, starting with "The Changeling", metric measures were used, albeit inconsistently. Later sequels of the show, such as Star Trek: The Next Generation, and Star Trek: Enterprise (set in the 22nd century), used the metric system exclusively.