Writing RPGs is Hard, Part 4: I’m Borrowing Not Stealing

Posted: 2022-06-08
Word Count: 1100
Tags: rpg traveller

One of my purposes for examining Cepheus Atom, Advanced Fighting Fantasy, and The Fantasy Trip was to examine some design principles for games I might write. Below are three I kind of prefer, found mainly in Cepheus Atom.

Definition By Differences

Maybe I talked about this obliquely before, e.g. “a blank sheet of paper is a valid character sheet”. The basic principle, though, is that the rules should define a PC or NPC by how they differ from an “average” character, not in absolute terms. This keeps the size and complexity of player character sheets down and allows GMs to more easily create NPCs on the fly.

For example, Fate Core defines PCs primarily through Skills (i.e. domains which receive modifiers to die rolls), Aspects (i.e. circumstances in which they can modify or reroll the dice), and Stunts (i.e. mechanical or narrative exceptions to normal rules). Absent any of those, the One Simple Rule is to roll the dice and interpret any total higher than a target number as a success. Compare to, say, the percentile system in RuneQuest and its successors, where the character sheet lists every commonly used skill, and a few uncommon ones, because the player has to know (or calculate) a default value for each.

The short character sheets for AFF2 and TFT still have to list a few basic numbers: SKILL, STAMINA, and LUCK for AFF2 or ST, DX, IQ, and MA1 for TFT. Cepheus Atom seems closest to this ideal: six Skills which one could assume are 0 if not listed and two randomly generated pools of points (Endurance and Lifeblood).

Roll-Over Mechanics

While one can make roll-under and roll-over mathematically equivalent, cognitively it’s easier to compare two numbers than to add and subtract. Roll-under typically means adjusting a target number then checking the dice; roll-over allows the GM to adjust a target number based on external factors while the player adjust their dice total based on their character’s factors, then compare the two. Roll-over also allows the GM to keep the final target number secret, when the player character wouldn’t necessarily know all the factors at play.

Cepheus Atom wins here in that every roll is 2d6 + modifiers at or above a target number. In TFT characters succeed if they roll 3d6 (sometimes 4d6, 5d6, or 2d6) at or below their DX (Dexterity) or IQ (Intelligence) attributes. AFF2 uses a similar roll-under mechanic with unopposed SKILL or MAGIC checks. In contrast, AFF2 handles combat by having all combatants roll at once, adding SKILL, “Special Skills” (if any), and circumstantial modifiers; combatants with the higher total hit their targets with a lower total. It’s easy to unify the two mechanics by making unopposed rolls 2d6 + SKILL + etc. vs a fixed 14, but that makes the minimum SKILL a few points higher than perhaps it has to be.

Orthogonal Abilities

The majority of RPGs define “base attributes”, derive numbers from them, and then add other abilities that modify those numbers. This implicitly links together all skills or activities covered under those attributes, which sometimes gets a little strained. For example, in Year Zero Engine games skills move from attribute to attribute between variants mainly so that one attribute won’t be more valuable than another. “Sense Emotion” or its equivalent is an Empathy skill in Mutant Year Zero and Vaesen, but a Wits/Mind skill in Forbidden Lands and Tales From The Loop. Also, stronger people are better craftsmen in Forbidden Lands and better mechanics in ALIEN. AFF2 and TFT has only two base attributes involved in success rolls, but even they have odd corner cases and exceptions.

A system like Fate Core is easier to comprehend and customize if it places possible tasks under orthogonal and independent “Skills”, defines characters directly through their proficiency in those Skills, and then defines other abilities that tweak probabilities in specific circumstances.

TFT fares worst, again, in that ST defines the amount of damage or fatigue the character can stand2, the types of weapons they can wield, and the chance of success for a few talents. AFF2 attributes are more orthogonal: STAMINA for damage and fatigue, LUCK for “saving throws” and tests for unintentional success, SKILL for tests of intentional mundane success, and MAGIC for magical chances of success3, plus derived MAGIC POINTS4 used only for spells. Cepheus Atom fares best: skills have non-overlapping domains and Endurance and Lifeblood absorb damage in a clearly delineated order.


From the above analysis it would seem that, of the three, Cepheus Atom would make the most solid basis for a custom system. It has the following desirable features:

  1. A character (PC or NPC) is defined solely by its Skill Ratings, i.e. modifiers to skill rolls to perform various tasks, and two (or fewer) point pools reduced by physical damage.

  2. The players and GM can determine success or failure by rolling dice (2d6 in Cepheus) and adding modifiers. In unopposed checks, the player (or GM?) compares the total to a fixed difficulty number; in opposed checks, two (or more?) parties compare against each other to determine the victor.

  3. A flat skill system avoids the problem of some attributes being more valuable than others. Granted, some skills may be more valuable than others, but in those cases players will simply avoid the less useful skills.

Some parts of AFF2, TFT, and other systems may be worth stealing:

I’ll detail the resulting system in a future post.

  1. Defined solely by the character’s species, but still important enough to write down. ↩︎

  2. For example, in TFT wizards burn Strength points to cast spells … so wizards have to be bodybuilders? ↩︎

  3. Certain mental Special Skills can use the higher of MAGIC or SKILL so as not to disadvantage magic-based characters. ↩︎

  4. Depending on the type of magic AFF2 magic-users burn MAGIC POINTS (derived from the MAGIC characteristic), STAMINA (normally lost by physical trauma), or, indirectly, LUCK (normally . ↩︎