player character's relationship with other characters and in turn the direction and outcome of the storyline. Later games in the series added several variations, including an action gauge that can be raised up or down depending on the situation, and a gauge that the player can manipulate using the analog stick depending on the situation. A similar type of conversation system later appeared in a more recent action role-playing game also published by Sega, Alpha Protocol. Another unique take on the concept is combining non-linear branching storytelling with the concepts of time travel and parallel universes. Early attempts at such an approach included Squaresoft's Chrono role-playing game series (1995–1999) and ELF's visual novel YU-NO: A girl who chants love at the bound of this world (1996). Radiant Historia takes it further by giving players the freedom to travel backwards and forwards through a timeline to alter the course of history, with each of their choices and actions significantly affect the timeline. The player can return to certain points in history and live through certain events again to make different choices and see different possible outcomes on the timeline. The player can also travel back and forth between two parallel timelines, and can obtain many possible parallel endings. The PSP version of Tactics Ogre featured a "World" system that allows players to revisit key plot points and make different choices to see how the story unfolds differently. Final Fantasy XIII-2 also features a similar non-linear time travel system to Radiant Historia. Early examples Early examples (pre-1983) of nonlinear gameplay include: Colossal Cave Adventure (1976) Zork (1977/1980) MUD1 (1978) Akalabeth (1979) Star Raiders (1979) Superman (1979) Temple of Apshai (1979) Computer Bismarck (1980) Flight Simulator (1979/1980) Mystery House (1980) The Prisoner (1980) Rogue (1980) Adventure (1980) 005 (1981) Bosconian (1981) Castle Wolfenstein (1981) Crush, Crumble and Chomp! (1981) Ultima (1981) Star Warrior (1981) Venture (1981) E.T. the Extra-Terrestrial (1982) Haunted House (1982) The Hobbit (1982) Pitfall! (1982) Raiders of the Lost Ark (1982) Snipes (1982) Time Pilot (1982) Ultima II (1982) See also Massively multiplayer online games Multiple endings Open world Metroidvania References Video game gameplay Video game terminology Emergent gameplay