In 1991, Emmett Brown built an 8-passenger DeLorean time machine that used Gale-Zemeckis coordinates to calculate time periods which could not be represented by the four-digit year system that was used by the time circuits. Later that year, Doc traveled to 2015 to explain how changing the past can have repercussions on the time stream. Biff Tannen's stealing the DeLorean time machine to travel to 1955 to give his younger self a Grays Sports Almanac was used as an example. He then showed a Gale-Zemeckis diagram which demonstrated the temporal changes.