"West Marches" is a player-driven style of RPG campaign. Rather than a single long storyline with the same group of adventurers, West Marches games are usually a series of distinct one-shots with a potentially ever-changing group of party members.

Ben Robbins, creator of the format, describes it on his blog, ars ludi:

Grand Experiments: West Marches

West Marches was a game I ran for a little over two years. It was designed to be pretty much the diametric opposite of the normal weekly game:

1. There was no regular time: every session was scheduled by the players on the fly.
2. There was no regular party: each game had different players drawn from a pool of around 10-14 people.
3. There was no regular plot: The players decided where to go and what to do. It was a sandbox game in the sense that’s now used to describe video games like Grand Theft Auto, minus the missions. There was no mysterious old man sending them on quests. No overarching plot, just an overarching environment.

The format is designed to avoid player apathy by involving players in scheduling and in deciding what to do. It is also intended to be adaptable to people with a variety of schedules. West Marches games do not necessarily need to have the exact same restrictions, but most of them do abide by most or all of these basic tenets to some extent.

The West Marches style was originally developed for use with D&D, but can also be adapted to a number of other similar RPGs.

Further information:

• Matthew Colville: The West Marches, Running the Game #30
• Jacob Kosman (Nerdarchy): Delving into the West Marches RPG Campaign Style
• Ken the DM - West Marches Repository (a brief summary of the format, as well as a number of relevant resources

See also: sandbox