Design Origins -- Part 4: The Core Mechanic

And we're back!

Today I want to talk about one of the more basic, mathematical conceits of the system, and how I eventually settled on it -- specifically, the "DC10" rule.

When you look at the basic resolution mechanics for d20 systems, it shakes out something like this:

Attack: d20 + ability modifier + proficiency bonus
Armor: 10 + ability modifier + armor bonus

My immediate thought was to strip away the extra modifiers, and just go with d20 vs. 10. This way, it keeps the math streamlined and balanced across the various classes in the game.

I also like the way that Apocalypse World has the 3 different ranges of results, based on 2d6. Similarly, TNP breaks down into miss/hit/crit, but based on the d20.

The fixed "DC10" rule also lends itself to using the Advantage/Disadvantage mechanic. Because the range of numbers does not have to be continually increased (with bigger/higher/more modifiers) to hit escalating target numbers, a mechanic like this can be used to tinker with probability more consistently and elegantly.

This also touches on one of the weird quirks of systems with increasing modifiers -- you end up with one of two outcomes: either the difficulty increases in a manner proportional to your capability increasing (thus keeping the actual math relatively flat), or; your capability increases, while the difficulty remains static (in which case... the game starts out hard and gets easier.)

If our goal is the former, then the easiest way to ensure that the math is as flat and balanced as possible, is to strip out modifiers where we can, and avoid inflating numbers -- which is ultimately what TNP ends up doing.

Another idea I borrowed from, was the 'feel' of the roll-under mechanic. Generally how this works within d20 systems, is you have your standard ability scores (generated by rolling 3d6, producing a range of 3-18.) Then, (to make a skill check, for example) you roll a d20 against the appropriate ability score, and try to "roll under" that score, in order to succeed.

What this means mechanically, is that having a higher score increases your odds of success. Most d20 systems use the conceit of "you want to roll high" thus making bonuses additive, and a little easier to wrap your head around; in a roll-under system, you actually want to subtract from your roll to increase your odds of success, thus your "bonuses" end up being negative numbers -- which is a little counter-intuitive.

But I liked the basic idea of rolling 'against' your number (in order to succeed) rather than rolling against the enemy's number. This helped to unify the mechanics of Attack/Defense/Skills within the designs; the PCs always roll, you always want to roll high, you add/reroll dice for your bonuses, etc.


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Next post should be up on August 8th, so check back then!