A video game with nonlinear gameplay presents players with challenges that can be completed in a number of different sequences. Each player may take on (or even encounter) only some of the challenges possible, and the same challenges may be played in a different order. Conversely, a video game with linear gameplay will confront a player with a fixed sequence of challenges: every player faces every challenge and has to overcome them in the same order.
A nonlinear game will allow greater player freedom than a linear game. For example, a nonlinear game may permit multiple sequences to finish the game, a choice between paths to victory, different types of victory, or optional side-quests and subplots. Some games feature both linear and nonlinear elements, and some games offer a sandbox mode that allows players to explore an open world game environment independently from the game's main objectives, if any objectives are provided at all.
A game that is significantly nonlinear is sometimes described as being open-ended or a sandbox, and is characterized by allowing players to measure progress through self-determined goals, independent of scripted game elements.
Level design
A game level or world can be linear, nonlinear or interactive. In a linear game, there is only one path that the player must take through the level, but in games with nonlinear gameplay, players might have to revisit locations or choose from multiple paths to finish the level.
As with other game elements, linear level design is not absolute. While a nonlinear level can give the freedom to explore or backtrack, there can be a sequence of challenges that a player must solve to complete the level. If a player must confront the challenges in a fixed order nonlinear games will often give multiple approaches to achieve said objectives.
A more linear game requires a player to finish levels in a fixed sequence to win. The ability to skip, repeat, or choose between levels makes this type of game less linear. Super Mario Bros. is an early example of this, where the player had access to warp zones that skipped many levels of the game.
In some games, levels can change between linear design and free roaming depending on the objective of the stage. Super Mario 64 is an example where the main stages are free roam, while the levels where Bowser is encountered follow a straight path to the end.
Open worlds and sandbox modes
When a level is sufficiently large and open-ended, it may be described as an open world, or "sandbox game", though this term is often used incorrectly. Open-world game designs have existed in some form since the 1980s, such as the space trading game Elite, and often make use of procedurally generated environments.
In a game with a sandbox mode, a player may turn off or ignore game objectives, or have unlimited access to items. This can open up possibilities that were not intended by the game designer. A sandbox mode is an option in otherwise goal-oriented games and is distinguished from open-ended games that have no objectives, such as SimCity, and Garry's Mod.
Branching storylines
Games that employ linear stories are those where the player cannot change the story line or ending of the story. Many video games use a linear structure, thus making them more similar to other fiction. However, it is common for such games to use interactive narration in which a player needs to interact with something before the plot will advance, or nonlinear narratives in which events are portrayed in a non-chronological order. Many games have offered premature endings should the player fail to meet an objective, but these are usually just interruptions in a player's progress rather than actual endings. Even in games with a linear story, players interact with the game world by performing a variety of actions along the way.