Nine Men's Morris is an abstract board game for two players. Players attempt to remove their opponents' stones, and the first player to completely deprive the opponent of stones wins the game.
Start of the game
The board has 24 spaces, arranged in three squares. Each player begins with 9 stones of their color in their reserve.
On a player's turn, a player either places one of their stones onto the board if there's any left, or otherwise moves one of their stones on the board. If a player moves a stone, that stone must be moved to an adjacent space. (See 'final three stones' below for possible exception.)
If by placing or moving this stone, the player forms a line of three of their stones, they have formed a "mill" and may remove any of their opponent's stones on the board. If it's possible, a player must remove a stone that is not a part of a mill, but if it's not possible, the player may remove any stone.
Final three stones: According to the chosen rule option, the last three stones must move to adjacent spaces (default for 'simple' games), or may move anywhere, not necessarily to an adjacent space (default for 'arena' games). This option can be changed by clicking the cog-wheel preferences icon next to the game picture on the 'Play now' page.
End of the game
The game ends if a player cannot make a legal move; that player loses. The game also ends if a player is reduced to two stones (with a loss for that player). This is because they cannot make any more mills, and so those stones will eventually be captured by the opponent, leaving no stones and thus no legal move to make.