Play by mail games are often referred to as PBM games, and play by email is sometimes abbreviated PBeM -- as opposed to face to face (FTF) games which are played in person. Another variation on the name is Play-by-Internet (PBI) or play-by-web (PBW). In all of these examples, player instructions can be either executed by a human moderator, a computer program, or a combination of the two.
In the 1980s, play-by-mail games reached their peak of popularity with the advent of Gaming Universal and Flagship magazine, the first professional magazines devoted to play-by-mail games. (An earlier fanzine, Nuts & Bolts of PBM, was the first publication to exclusively cover the hobby.) Bob McLain, the publisher and editor of Gaming Universal, further popularized the hobby by writing articles that appeared in many of the leading mainstream gaming magazines of the time. Flagship later bought overseas right to Gaming Universal, making it the leading magazine in the field. Flagship magazine was founded by Chris Harvey and Nick Palmer (now an MP) of the UK. The magazine still thrives, albeit under a different editor over twenty years later.
In the late 1990s, computer and Internet games marginalized play-by-mail conducted by actual postal mail, but the postal hobby still exists with an estimated 2000–3000 adherents worldwide.
In the case of a two player game such as chess, players would simply send their moves to each other alternately. In the case of a multi-player game such as Diplomacy, a central game master would run the game, receiving the moves and publishing adjudications. Such adjudications were often published in postal game zines, some of which contained far more than just games.
The commercial market for play-by-mail games grew to involve computer servers setup to host potentially thousands of players at once. Players would typically be split up into parallel games in order to keep the number of players per game at a reasonable level, with new games starting as old games ended. While the central company was responsible for feeding in moves and mailing the processed output back to players, players were also provided with the mailing addresses of others so that direct contact could be made and negotiations performed. With turns being processed every few weeks, more advanced games could last over a year.
Game themes are heavily varied, and may range from those based on historical or real events to those taking place in alternate or fictional worlds.
Inevitably, the onset of the computer-moderated PBM game (primarily the Legends game system) meant that the human moderated games were pushed into the "non-profit-making sector" of the industry.
Actual move/turn submission is traditionally carried out by filling in a turn card. This card has formatted entry areas where players enter their planned actions (using some form of encoding) for the upcoming turn. Players are limited to some finite number of actions, and in some cases must split their resources between these actions (so that additional actions make each less effective). The way the card is filled in often implies an ordering between each command, so that they are processed in-order, one after another. Once completed, the card is then mailed (or, in more modern times, e-mailed) to the game master, where it is either processed, or held until the next turn processing window begins.
By collecting turn cards from a number of players and processing them all at the same time, games can provide simultaneous actions for all players. However, for this same reason, co-ordination between players can be difficult to achieve. For example, player A might attempt to move to player B's current location to do something with (or to) player B, while player B might simultaneously attempt to move to player A's current location. As such, the output/results of the turn can differ significantly from the submitted plan. Whatever the results, they are mailed back to the player to be studied and used as the basis for the next turn (often along with a new blank turn card).
While billing is sometimes done using a flat per-game rate (when the length of the game is known and finite), games more typically use a per-turn cost schedule. In such cases, each turn submitted depletes a pool of credit which must periodically be replenished in order to keep playing. Some games have multiple fee schedules, where players can pay more to perform advanced actions, or to take a greater number of actions in a turn.
Some role playing PBM games also include an element whereby the player may describe actions of their characters in a free text form. The effect and effectiveness of the action is then based on the judgement of the GM who may allow or partially allow the action. This gives the player more flexibility beyond the normal fixed actions at the cost of more complexity and, usually, expense.
Some sites have extended this gaming style by allowing the players to see each other's actions as they are made. This allows for real time playing while everyone is online and active, or slower progress if not.