Design Patterns: Perpendicular Constraints

There’s a useful term in project management called the Critical Path, which is defined as the least amount of time needed to complete some multi-step operation. A project manager tries to figure out how to sequence tasks in a way that is most efficient and wastes the least amount of time. Eurogames offer a similar challenge to players. In race-style games, players try to cross the finish line in the fewest number of actions, while in games with turn-based end conditions players try to squeeze the most points out a relatively fixed number of turns.

Cooperative games incorporate the Critical Path concept too, but we’d state it a bit differently. The critical path can be thought of as the task players must accomplish to win the game. In Pandemic, for example, players must cure all the diseases in the game in order to win. Thus, the actions that lie on the critical path are the actions that involve moving cards between players to create sets that can be turned in for cures.

There’s a tension between clarity and complexity when it comes to building critical paths. On the one hand, we want players to know what they’re trying to achieve, to understand how the actions at their disposal will get them closer to that goal, and how they need to sequence those actions. On the other hand, if those ideas are too simple and too clear, the game is boring and lacks agency. Once you’ve solved for the critical path and you know the shortest way to get from A to Z, there’s not much game left. But making the critical path complex or confusing doesn’t really help either. It creates frustration, not interesting play.

What many cooperative games do is introduce what I call Perpendicular Constraints. These are game systems that interrupt and cut across the critical path, forcing players to distract themselves from their core mission to deal with some other matter. In cooperative games, most of the ways that players can lose the game are perpendicular constraints.

Going back to Pandemic, players want to move cards around and cure diseases, but they also have to treat disease cubes and remove them from the board, or they’ll soon lose the game to too many outbreaks. However, traveling to different parts of the board often requires spending cards – the very same cards that are needed to cure the disease! Here, players face the dilemma of a good perpendicular constraint: to spend resources to address an immediate crisis, or to accept the costs of not addressing it and save the resource to advance down the critical path instead.

At one end, a truly perpendicular constraint doesn’t advance you at all down the critical path towards victory. But not all constraints are purely perpendicular, and not all methods of addressing constraints are single-purpose. For example, Research Stations in Pandemic improve travel, helping players get to hotspots that require treatment, and also helping players meet at cities to trade cards. Research Stations also provide players with another location for trading in sets to cure diseases. The action of building a Research Station helps players resolve perpendicular constraints, while also potentially advancing down the critical path.

The Forbidden series, also by Matt Leacock, is in some ways more elegant and tightly coupled than Pandemic as regards its perpendicular constraints. In Pandemic, disease cubes need treatment because the game declares, by fiat, that cities can hold no more than three disease cubes, and that adding more instead creates an outbreak, and some number of outbreaks will lose the game. But in Forbidden Island and Forbidden Desert, instead of the disease cube mechanism, the game’s locations themselves either sink or get buried, respectively. As tiles become inaccessible, players are limited in the routes they can use to travel the board, and they must also take care lest the artifacts they must recover to win become completely unreachable. In a sense, the work of shoring up or clearing tiles is less perpendicular to the overall goal.

One feature of perpendicular constraints in co-op games is that they are usually maintenance tasks, a kind of tactical chore that, if ignored, will soon overwhelm the players. They don’t in any way confuse or obscure the critical path, they simply interfere with players’ ability to accomplish it by consuming resources. From a design perspective this is quite attractive, since it gives players challenging and meaningful decisions about how to spend their actions and resources, while keeping the game intelligible. The difficulty of the decision doesn’t come from a complicated mechanism, but rather because each decision is a wager, a question of how much risk are the players willing to tolerate.

Done right, perpendicular constraints help shape and define a game,, and provide depth, meaning and agency to player decisions. But look out! Constraints that don’t have short enough triggers or strong enough consequences can be ignored, or perhaps treated as optional side-quests. Constraints that are too parallel to the critical path present dominant strategies instead of interesting choices. A good design finds just the right balance among these concerns.

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