In video games and other games, the passage of time must be handled in a way that players find fair and easy to understand. This is usually done in one of the two ways: real-time and turn-based.
Real-time
Real-time games have game time progress continuously according to the game clock. One example of such a game is the sandbox game Terraria, where one day-night cycle of 24 hours is equal to 24 minutes in real time. Players perform actions simultaneously as opposed to in sequential units or turns. Players must perform actions with the consideration that their opponents are actively working against them in real time, and may act at any moment. This introduces time management considerations and additional challenges (such as physical coordination in the case of video games).
Real-time gameplay is the dominant form of time-keeping found in simulation video games, and has to a large degree supplanted turn-based systems in other video game genres as well (for instance real-time strategy). Time is an important factor in most sports; many, such as soccer or basketball, are almost entirely simultaneous in nature, retaining only a very limited notion of turns in specific instances, such as the free kick in soccer and the free throw and shot clock in basketball. In the card games Nerts and Ligretto, players must compete to discard their cards as quickly as possible and do not take turns.
While game time in video games is in fact subdivided into discrete units due to the sequential nature of computing, these intervals or units are typically so small as to be imperceptible.
Turn-based
Turn-based games have game flow that is partitioned into defined parts, called turns, moves, or plays. A player of a turn-based game is allowed a period of analysis (sometimes bounded, sometimes unbounded) before committing to a game action, ensuring a separation between the game flow and the thinking process, which in turn presumably leads to better choices. Once every player has taken their turn, that round of play is over, and any special shared processing is done. This is followed by the next round of play. In games where the game flow unit is time, turns may represent periods such as years, months, weeks or days.
Turn-based games come in two main forms depending on whether, play is simultaneous or sequential. The former games fall under the category of simultaneously executed games (also called phase-based or "We-Go"), with Diplomacy being a notable example. The latter games fall into player-alternated games (also called "I-Go-You-Go", or "IGOUGO" for short), and are further subdivided into (A) ranked, (B) round-robin start and (C) random—the difference being the order under which players start within a turn: (A) the first player being the same every time, (B) the first player selection policy is round-robin, and (C) the first player is randomly selected. Some games also base the order of play on an "initiative" score that may in part be based on players' attributes or positions within the game or other, outside factors as well as dice rolls. Wizard101 is an example of this style.
The term turn-based gaming is also used in play-by-mail games and use browser-based gaming sites that allow for gameplay to extend beyond a single session, over long periods of time, often requiring months to complete games such as Go, chess, and infinite chess.
Sub-types
Various adaptations of the real-time and turn-based systems have been implemented to address common or perceived shortcomings of these systems (though they often introduce new issues that did not exist before). These include:
Timed turns and time compression
Timed turns are designed to resolve issues of fairness where one player uses a greater amount of time to complete their turn than another player. In chess, for instance, a pair of stop clocks may be used in order to place an upper limit on game length.
In exchange chess, four players on two teams play on two boards with each team taking one white and one black side. Any taken piece is given to a teammate, and can be placed on their board as a standard move (in any position that does not put their opponent in check). A common strategy is to gain a temporary material advantage, pass it on to a teammate, and then stop playing on one's own board—thereby allowing the teammate to use the advantage for many future moves on their board. To avoid this, players are often limited to ten seconds per move—with their opponent being allowed to remove one of the player's pawns from the board for each additional ten seconds consumed.