ABACUS is built to be an ultralight system that allows for a high degree of detail and flexibility. Put simply, it attempts to do everything for everyone without becoming too much of a burden, in part due to modularity and a universal design principle that focuses on parallelism and an easy to understand structure inspired by grammar and linguistics.
One of the core elements of language is the consideration that there is an appropriate syntax. Unfortunately, what we see in a lot of games is that they’re not designed with this in mind. For instance, in English we put the subject first, then the verb, then an object. We do this in every sentence we speak, with the object being optional, but we’ve got a number of additional criteria.
First I should define the concepts of a sentence-based approach to game design:
An “Impetus” is a set of required criteria for an action to take place. This can be inferred (i.e. a player is using a character), or explicit (a character has X of Y). These are rare in traditional tabletop games’ “play”, though we can see them in Vancian spellcasting and daily abilities. They’re more common in CCG’s or Video Games, though if we want to go to something like character development they’re pretty common place. They are conditions that are relied upon for an action to occur.
A “Stimulus” is the actual effort by a character to do something. This is usually where an external input, such as dice or cards, are used to determine something, and it’s where a player declares an action. Stimuli are things like “I swing my axe at the rat!” or “I activate Flurry of Blows”, where an action is beginning occur. Stimuli don’t necessarily have to be up to chance either, nor do they have to be explicitly called; they’re where outside information comes into the “sentence”.
Finally, a “Reaction” follows the events that occur; typically they’re related directly to a stimulus, though there are cases of reaction chains (hits in a war-game resulting in an ammunition explosion that does X damage to nearby foes based on their Y). These are where game values are changed; characters lose health, gain a temporary or permanent bonus, and the narrative is advanced, or
ABACUS is essentially inspired by making everything a simple sentence. If an “action” is to be compared to a sentence, then there are a number of parts. The traditional structure for stuff like this in games is that an action is a “Stimulus-Reaction” chain. However, many of these games then go on to complicate certain rules unnecessarily. For instance, a single combat action in the d20 system is “Stimulus-reaction>stimulus-reaction”, with the first stimulus being the attack roll, a reaction of comparing the result of the roll to the defender’s Armor Class. Should the reaction be favorable, then another die is rolled, and the target loses health, dies, or absorbs all the damage with a resistance or DR. In Shadowrun, the chain for casting a spell is typically “Stimulus-reaction>stimulus-reaction, stimulus-reaction”, with the first stimulus being the roll made by a spellcaster leading to a reaction of the spell failing or succeeding based on the caster’s ability, then there’s the potential for the target to resist, which may lead to partial or complete negation of the spell’s effects. Once this is done, there’s another stimulus-reaction chain for resisting Drain resulting from the spell.
ABACUS uses only three chains ever; Impetus-Stimulus-Reaction, Impetus-Impetus-Reaction or Stimulus-Reaction, and the former are rarer. There are no cases where stuff occurs conditionally; if an action is begun it can always be carried out to completion, even if the completion has no effect.
Impetus-Stimulus-Reaction action chains are pretty rare, occurring primarily in the financing of Dominance abilities in Orchestra. This is when an ability feeds into a condition that is variable, such as a pool of Resolve being used to determine whether or not a power can be financed effectively. These are otherwise identical to Stimulus-Reaction chains, except that preexisting conditions do not merely play into the stimulus but must also fall into certain criteria.
Impetus-Impetus-Reaction action chains occur most notably in the case of the Forgery system in Ostravia; they are the aftermath of a “pool test”, and compare one result to another, based on adding up pre-existing values and then comparing them in the reaction phase.
Stimulus-Reaction actions are the most common, and are the “common” actions that guide the majority of events in the game. They’re used when we need to look at something happening: can someone climb a tree? We use their ABACUS attributes, skills, and specializations, add it all together, roll the dice, then move into the reaction phase where we compare the difficulty and the roll to see what happens.