Game design is the craft of placing together patterns into a game text so that the players of the game will produce these patterns in their play. Good game design is to ensure that these patterns interact well, and produce the sort of play desired by the designer. Because of this, patterns which people can quickly understand and absorb are extremely valuable design tools. One type of pattern are finite structures, such as stacks and queues. I introduced these structures here, and this month I am further developing one of these structures, the stack, as a core for game design.

Stacks and Queues

A stack is a sequence where each new piece is placed on top of the previous ones. And the removal of pieces only happens from the top down. This means you can only access the stack from the most recent piece, the earliest additions are inaccessible until everything which occurred after them is removed.

On the other hand, queues are sequences with addition and removal happening at opposite ends. Queues cause a flow of pieces from the input to the output, introducing a delay, but not changing the order. In a queue the choice of when to remove or when to add a piece only affects the delay, not the order.

But in a stack, the decision to add or remove makes a significant difference. This is ultimately the power of a stack in game design, each decision can have a long term consequence, and that consequence is compounded by the successive decisions. But at each step the decision need only be whether to add or remove.

Stacks as Postponement

Consider an intrigue drama game with a mechanic where you can choose to stall fate. Each time a player tries to move towards their goals they include a risk of some adversity, however if they fail their roll the adversity can either be placed onto the stack or dealt with immediately. Likewise, on a positive outcome the player can choose to introduce some effect from the top of the stack as a complication. In this way, the PCs control most of the scenes, passing control among them. After each stacking, the GM rolls 2d6, and if the stack contains more outcomes than the number rolled, then the GM introduces a Crash and Burn scene, where all of the outcomes come off the stack and occur at once.

Beth has decided to interrogate the thug she just captured, if she succeeds she wants to know where the villain's headquarters is, if she loses she wants the thug to escape and capture her. The GM rules that these are equal stakes. And the adversity stack looks like this:

• Wild Goose Chase
• Enemy Reinforcements
• Faulty Equipment
• Corrupt Police

She rolls and succeeds, but she decides that the adversity stack is getting a little too full, so she decides to take the Wild Goose Chase from the top, meaning that the thug spills the beans, but his information is faulty, so when the team gets to the location, they find only an abandoned warehouse. Had she failed, she could have placed the adversity of being captured on the stack, or just taken the result, and let herself be captured. The former would cause more difficulty for her teammates, but the later would make a Crash and Burn scene fairly likely.

Stacks as Reaction

Another application of a stack structure is as a means of reaction. By letting attempts be placed on a stack before they occur, the reactions to these attempts can be placed above them, ensuring that they must be removed before the attempt can be resolved. This mechanic can be seen in variety of games, including card games and RPGs, but in general is used to make attempts more tactical.

Each decision, whether to remove or add can be very important. The unfortunate side effect of this importance is that these stack systems often become very complex. Complexity increases by keeping these stacks abstract, rather than having a tangible or visual stack to reference.

This principle of reaction can be taken to an even greater extent, by building the entire social structure of play as proposal and reaction. Most game worlds are developed to focus onto the precise situation of the player characters. A stack makes this focus concrete, by adding layers of specification, until a level is resolved in the course of the game. The lowest of these levels may be very broad, such as "Europe", beneath "the 100 year war", beneath "South of France", down to a specific event or location at the top. Adding to the stack heightens the risk, and removing items requires confronting some portion of that risk.

Stacks as Priorities

Rather than communal stacks, a stack can be an intrinsic part of each character. Stacks of beneficial resources produce a very intriguing gradient. Items at the top of the stack are easiest to use, but are easiest to lose. Those at the bottom only matter when resources are low. The decisions for how to build such a stack must be based on what your character views as trivial and common versus what is treated as a last resort.

This dichotomy between casual and critical can be wonderfully descriptive. Consider designing a character based on a stack of goals or beliefs. Each time a goal or belief on the stack is about to be ruined or violated, the player may override the effect, by spending the top one on the stack. Over time more may be added, but in the worst situations only the most treasured hopes and principles will persist, with everything else sacrificed to achieve them.

This same sort of structure can be used as more than just a personality mechanic, indeed, next month I will be presenting Vitae, a RPG design where the stack is the core mechanic.

Next Month: Bloody Decisions