Time-keeping systems

In video 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 two ways, or a combination thereof: real-time and turn-based.

Real-time
In real-time games, game time progresses continuously according to the game clock. 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; and 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 Nertz 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 to the player.

Turn-based
In turn-based games, game flow is partitioned into well-defined and visible parts, called turns. 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 his or her 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 such things as years, months, weeks or days.

Turn-based games come in two main forms depending on whether, within a turn, players play simultaneously or take their turns in sequence. The former games fall under the category of simultaneously-executed games (also called phase-based or "We-Go"), with Diplomacy being a notable example of this style of game. 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. Power Grid is an example of this style.

The term turn-based gaming is also used in Play-by-mail games and to refer to browser-based gaming sites that allow for game-play to extend beyond a single session, over long periods of time—often taking months for complex games like Go or chess to finish.

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
 * Time compression
 * Tick-based
 * Active Time Battle
 * Simultaneously-executed
 * Clock-based
 * Initiative-based
 * Pausable real-time

Special turns and phases
In some turn-based games, not all turns are alike. The board game Imperium Romanum II (1985), for instance, features a "Taxation and Mobilization" phase in every third turn (month), which does not occur in the other turns. In the board game Napoleon (1974), every third player turn is "night turn" where combat is not allowed.

Other turn-based games feature several phases dedicated to different types of activities within each turn. In the Battle Isle (1991–2001) series of video games players issue movement orders for all units in one phase, and attack orders in a later phase. In the board game Agricola (2007) turns are divided into three phases: "Upkeep", "Replenishing" and "Work". A fourth phase, "Harvest", occurs every few turns.

Partially or optionally turn-based and real-time
Many other games that are not generally turn-based retain the notion of turn-based play during specific sequences. For example, the role-playing video games Fallout: A Post Nuclear Role Playing Game (1997) and Silent Storm (2003) are turn-based during the combat phase, and real-time throughout the remainder of the game. This speeds up portions of the game (such as exploration) where the careful timing of actions is not crucial to player success. Some turn-based games have been criticized for omitting this feature.

Other video games, such as the Total War series (2000–2011), X-COM (1993) and Jagged Alliance 2 (1999), combine a turn-based strategic layer with real-time tactical combat or vice versa.

Lastly, the video games X-COM: Apocalypse (1997), Fallout Tactics (2001) and Arcanum: Of Steamworks and Magick Obscura (2001) offered players the option to play in either turn-based or real-time mode via a configuration setting. The latter also offered a "fast turn-based" mode, though all three of the game's modes were criticized for being poorly balanced and overly simplified.

Real-time vs. turn-based gameplay
A debate has emerged between fans over real-time and turn-based video games (usually some type of strategy or role-playing game) based on the merits each system.

Various arguments are made by proponents. Arguments made in favor of turn-based systems include:


 * Players are able to plan their moves to a greater degree given the extra time available to them, allowing game designers to cater to these players by offering additional tactical and gameplay options. The same options when used in combination with the time-pressures of real-time games, on the other hand, can cause new players to feel overwhelmed.
 * Games are more fair due to a lack of reliance upon player reflexes. A player with slower reflexes is not at a disadvantage compared to faster players; rather, only the ability to think through and solve the current problem is important.
 * Games can in theory have better artificial intelligence due to the greater amount of computer processing power available to them.
 * It is more realistic to control multiple units intelligently using this system, as players do not have to divide their attention among multiple independent units all moving simultaneously. Likewise, it is easier to keep track of what the enemy is doing at all times since the player is typically informed of every move in advance (not taking into account fog of war).

Arguments made in favor of real-time systems include:
 * Armies pausing mid-combat to take turns and act in a sequential manner is unrealistic. Real-life combat occurs simultaneously with no side pausing to let the other side move; however, this only pertains to sequential turn-based systems, not "we-go" systems.
 * Thinking (and acting) quickly is part of the strategy and constitutes an additional element of challenge.
 * Real-time systems are viscerally exciting and add to players' sense of immersion.  Players feel more like they are really "there" and experiencing game events first-hand.
 * Turn-based games have too many rules and are difficult to master.
 * Real-time games are more multiplayer-friendly. Sitting around and waiting while other players take their turns can become tiresome, and can be abused as means of retribution against a winning player.
 * The added element of a shared clock ensures that each situation cannot be reduced to an easily repeatable sequential series of steps. Rather, the reliance upon player timing introduces an element of chaos and ensures that outcomes are highly variable.