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A game demo is a (usually) freely distributed piece of an upcoming or recently released video game. Demos are typically released by the game's publisher to help consumers get a feel of the game before deciding whether to buy the full version and/or keep it.
In the early 1990s, shareware distribution was a popular method for publishing games for smaller developers, including then-fledgling companies such as Apogee Software (now 3D Realms), Epic MegaGames (now Epic Games), and id Software. It gave consumers the chance to try a trial portion of the game, usually restricted to the game's complete first section or "episode", before purchasing the rest of the adventure. Racks of games on single 51⁄4" and later 3.5" floppy disks were common in many stores, often very cheaply. Since the shareware versions were essentially free, the cost needed only the covering of the disk and minimal packaging. Sometimes, the demo disks were packaged within the box of another game by the same company. As the increasing size of games in the mid-90s made them impractical to fit on floppies, and retail publishers and developers began to earnestly mimic the practice, shareware games were replaced by shorter demos that were either distributed free on CDs with gaming magazines or as free downloads over the Internet, in some cases becoming exclusive content for specific websites.
There is a technical difference between shareware and demos. Up to the early 1990s, shareware could easily be upgraded to the full version by adding the "other episodes" or full portion of the game; this would leave the existing shareware files intact. Demos are different in that they are "self-contained" programs which are not upgradable to the full version. A good example is the Descent shareware versus the Descent II demo; players were able to retain their saved games on the former but not the latter.
Magazines that include the demos on a CD or DVD and likewise may be exclusive to a certain publication. Demos are also sometimes released on cover tape/disks, especially in the United Kingdom and mainland Europe, but given the increasing size of demos and widespread availability of broadband internet, this common practice throughout the 1980s and '90s gradually lost cover focus to full games. With the advent of console online services such as Xbox Live or PlayStation Network, demos are also becoming available as a free or premium download.
Console manufacturers also often release their systems with a demo disc containing playable previews of games to be released for their console.
The availability of demos varies between formats. Systems that use cartridges typically did not have demos available to them, due to the cost of duplication, whereas systems supporting more cheaply produced media, such as tapes, floppy disks, and later CD-ROM and DVD-ROM have; the Internet has more recently been a source for demos, although typically this is in addition to other distribution media available for the system in question. Also the Xbox Live as well as other gaming services have demos available for download 24/7.
Game demos come in two variations: playable and non-playable (also called a "rolling demo"). Playable demos generally have exactly the same gameplay as the upcoming full game, although game advancement is usually limited to a certain point, and occasionally some advanced features might be disabled. A non-playable demo is essentially the gaming equivalent of a teaser trailer.
Generally, playable demos are stripped down versions of the full game, restricting gameplay to some levels, only allowing access to some features, or limiting the amount of time playable in the game.
However, some demos provide content not available in the full game. An example of this was the Age of Empires demo which included a Hittites campaign and two maps not available in the full version. Also, the Half-Life demo Half-Life: Uplink is a self-contained game, adapted from material cut from the development of the main game. The demo for "The Stanley Parable" takes place in an area created specifically for the demo to show off the premise and humor of the game, as the narrator states multiple time that the player is taking part in a "video game demonstration".
In other cases a demo may differ from the equivalent section in the full game, for instance when the demo is released as a preview before the full game is completed. An example of this is the demo for Mafia II which took place in an altered version of the Buzzsaw mission set in the 1950s, as opposed to the equivalent mission in the full game, which was set in 1945.
Demos for platform or other action games generally only include the first few levels of the game. Demos of adventure games are often limited to a very small number of rooms, and have the "save game" feature disabled. Demos of sports games usually limit play to an accelerated half-time or complete match between a small number of teams (which at the same time led to the practice of "demo expanders" that allow the tweaking of some of those settings). Likewise, demos of racing games are ordinarily restricted to a single race with a pre-selected car.
A non-playable demo is a recording of gameplay, either recorded in a video, or played through using the game's own engine showing off the game's features. They are mainly displayed at gaming conventions, such as E3, when the game is still in early production as a technology or gameplay preview. Such demos might also be distributed through the Internet or with magazines as trailers for an upcoming game, or featured at retail stores (often among playable demos).