RPG Mechanics

By far the most difficult part of attempting to craft an RPG is deciding on the resolution system, aka the ‘mechanics’. I’ve encountered many varieties of systems using different dice, cards or other ways of resolving issues. There are systems which rely on narration and don’t have dice all the way up to massively crunchy systems where there’s a dice and chart for almost everything!

Part of the reason for writing this is to help me flesh out and clarify what makes each system unique, and analyse their strengths.

Dice System

Generally there’s 2 main approaches involving dice in an RPG; a pool or a variable target number. The major approaches are succinctly covered here.

Dice pool methods involve rolling a number of dice depending on the situation and counting some results (say all values over 5) as successes. Depending on the specific system, a player may require as few as one success or perhaps as many as four or five are required to mean total success of a task. Some systems allow for varying the target number -so in a d6 pool 4+ might be a success in a normal task, whereas only 6+ are successes for a very difficult task. Others change the number of successes required to moderate the difficulty but leave the value for success untouched. An example of games that use this are Vampire/WoD (d10), Shadowrun (d6), and Powered by the Apocalypse (PbtA – d6).

The other general system, variable target number, is again determined by rolling one or more dice which are (usually) summed along with modifiers and then the total compared to a value. The most well known example is D&D; a player rolls a d20 adds modifiers and the total is compared to a target number (such as armor, or a spell’s save difficulty).  If it equals or beats the target number the sword hits or the action succeeds. This has the advantage that bigger numbers are generally better (a +9 to hit is much better than a +4 to hit) and that modern game theory ensures that sane control is kept over the target numbers. (Bounded accuracy!)

Variations on this method include percentile systems (d10/0) which again are calculated with modifiers and compared to values (often the users abilities), a 3d6 system (Song of Ice & Fire or Dragon Age) where you simply sum the dice and compare to a skill or value as well as the FATE system which uses bespoke dice (+1,+1, +0, +0, -1, -1) known as dF, to give a  nicely bell curved range of results from -4 to +4, with +0 followed by +1 or -1 being the most common occurrence.

There are also hybrid systems – the one that springs to mind is Mophidius’ 2d20 system which combines a pool of d20s (not always 2!) compared to a derived target number. While the system has it’s own issues – it seems to draw a lot from FATE in terms of feeling but implements it with a pool system – it is an example of both a pool based system combined with a variable target system.

Other Bits

However dice alone do not a great game make. Without classes or races or equipment you couldn’t generate the numbers required for D&D to run smoothly. By way of contrast, dice pool systems often tend to lean away from classes and equipment for a less detailed but more fluid skill based narrative system.

My main concern with a d20 or even percentile based system is that there are a lot of details and modifiers which I would not only need to write, test and balance but that players would also need to record and remember. An example that springs to mind Starfinder by Piazo. On the surface is this is another d20 system similar to D&D, but it has an almost impenetrable level of class features, feats, and abilities to the point where I struggle to imagine how I would ever play it, let alone GM a game.

In Starfinder there are a myriad of dense options for each class which make the game quite ‘crunchy’ which I’m sure appeals to some, but makes me concerned because you’re constantly having to reshuffle your numbers every time you get new equipment or abilities. Equipment is also level scaled so it is necessary to be constantly upgrading gear or risk falling behind.

Contrast this approach with 5E D&D where the numbers are more restricted due to ‘bounded accuracy’, where character options are more streamlined (restricted?) and you’ve got a system which is easy to pick up, scales well and doesn’t require massive amounts of bookkeeping.

Because the system is well established the game is easier to run which to me contributes to the most important thing  about any game – playability.

Dice Size

Another question is which dice to use? Rolling dice is a very tactile experience and in a game that occurs ephermally it might be the only physical ‘component’. Certain dice are easier to roll in large numbers (d6, d20s) whereas others are quite difficult to roll en masse or read (d4s, d8s).

D20 – have a wide range of values allowing modifiers to be calculated according to how much impact you want them to have in 5% increments.

d12 – unfortunately there’s very little  that a d12 brings mathematically that a d6 or d20 doesn’t really bring. I guess unless you want to go for a 7/12 ratio for success or something, there’s really very little reason to use a d12. There is a Lord of the Rings system that has a special d12 ranging from 1-10 with an evil and good symbol – presumably some sort of special effect.

d10 – have the advantage of being decimal, being easy to roll in a pool system and by being in 10% increments makes the maths easier to calculate

d6 – easy to roll but with limited outcomes and so needing a number (2d6,3d6) to distribute out but at that point you might as well use a d20? Great for pool systems though where more dice = better.

dF- hard to find (and expensive) but providing a nice distribution of results tending around +/-1 or 0.


I am strongly leaning to a pool based system since it is less ‘blow by blow’ and can be a slight bit more abstract giving players and GMs room to forge story elements without being too constrained. I do like the FATE system generally since it’s highly narrative although logicistally the bespoke dice can make the game hard to access. A variant D6-D6 system might be an option (ranging from -5 to +5) but the numbers aren’t as good as 4dF.

PbtA systems like Blades in the Dark seem to strike the balance between adding dice and pooling them, but Blade hacks need ‘playbooks’ to be written and defined. While I’m not shying away from work, I’m concerned that it pigeonholes players into stereotypes which defeats the point of being able to ‘hack’ and redefine your character.

To that end I’m leaning towards a classless/skill based system (like Vampire) with narrative elements (like FATE) – so right now a toss up between PbtA (of which I have very little actual play experience) and FATE. The plus side is that these systems are both ‘open source’ which is a great thing in terms of RPG advancement.