The core mechanism, the backbone, for almost all types of board games is some kind of action selection mechanism. These days, many board games can be classified as worker placement games. In these games, players take actions from a common action pool, all players usually have equal access to the action pool, and each action taken usually benefits only that player who took the action.
One of my all-time favorite games, Puerto Rico, does things a bit differently. The backbone of the game in this case is a role-selection mechanism, in which the roles are chosen from a common pool. Unlike most worker placement games, the action dictated by the role is taken by all players. What Puerto Rico's role-selection mechanism does have in common with many worker placement games, however, is that the action pool stays more or less the same during the whole game.
In Spring 2008 I was thinking of different ways to implement action selection in board games. After some pondering, I wanted to try the following idea: Each player has a personal action pool – with the actions being represented by action cards – that no other player can access. In addition, all players share a common action card pool and can swap one of their personal action cards with one of these shared cards. An action card can be activated only from your personal pool, and a player's hand size is limited to two action cards. As a result, the common action card pool will change constantly, and players will have to think carefully which cards to hang onto and which to pick up.
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