The "cover system" is a gameplay mechanic used in first-person and third-person shooter or stealth-action games. It involves the playable character using an object in the environment as cover from enemies, as well as a concealed position from which the player can return fire.
Arcade light-gun shooters
The first 3D game with a cover mechanic was Namco's 1995 first-person, light-gun shooter, arcade game Time Crisis, which introduced a dedicated cover button, specifically an "action" foot pedal, that could be used to take cover behind in-game objects. This cover mechanic helped the Time Crisis series distinguish itself from rival light-gun shooters, like Sega's Virtua Cop, and took advantage of the players' hand-foot coordination to create a new arcade game experience.
In 2000, Police 911 introduced a unique cover system where the player takes cover by physically ducking for cover (detected by motion-sensing technology) rather than pressing a button. A more recent arcade variation of the cover system is the 2007 shooter 2 Spicy, where players use foot pedals to move from one destructible cover to the next, and which allows to players to face-off against each other using such a cover system.
While Metal Gear Solid was one of the first third-person games to allow you to press up against walls, it was WinBack: Covert Operations, released in 1999, which was the first third-person game to allow you to pop out of and shoot around a piece of cover. This mechanic was refined in Metal Gear Solid 2: Sons of Liberty, where Snake or Raiden are able to take cover behind walls, boxes or crates and pop out to shoot at enemies, while the improved enemy AI allowed enemies to also take cover from the player character; the enemies would often take cover to call for backup, and during battle, they would take cover then pop out and shoot at the player or blindly throw grenades from behind their cover.
This mechanic was developed further in Killswitch which allowed the player to transition between pieces of cover, throw grenades and blindfire. Gears of War later refined this mechanic and was very well received by the gaming press, to the point that nearly every third-person shooter released after Gears of War has incorporated this mechanic. Cliff Bleszinski, Gears of War's Lead Designer, has said that the act of moving between pieces of cover was very much inspired by swinging between platforms in Bionic Commando.
Vanquish attempted to reinvent the cover system, by having bullets and missiles coming from all directions in a manner reminiscent of bullet hell shooters and with cover being easily destroyed, forcing the player character to be on the move, while the game also penalizes the player on the scoreboard for the amount of time spent in cover. Its slide-boost mechanic also allows the player to slide-boost into and out of cover at high speeds, acting as both a defensive escape and an offensive setup, increasing the pace and opening up new gameplay possibilities for cover-based shooters.
In first-person shooters, the view sometimes changes from first-person to third-person while the player moves into cover. Examples of this are the Rainbow Six: Vegas games, Brothers in Arms: Hell's Highway and the James Bond shooter Quantum of Solace. Killzone 2 features a cover system, but the camera remains in first-person view the entire time.
Although critics often praise games for including cover systems, many hardcore first person shooter players find them unbalanced. Cover systems give players the unnatural ability to see around corners, giving those taking cover an advantage over those who are mobile. These systems also encourage camping of spawn areas and choke points, as players can simply pop out and shoot anybody trying to pass.