Monday, February 2, 2009

Principles of RPG design


I've been thinking and reading a lot about good game design, and I think I've identified a few principles that should be present in a good game.

- Rule of Three
- Magic Sevens
- Granularity
- Odds of success
- Simple Math
- Essential Elements of D&D


Good things tend to come in Threes. Check out Delta's article on the subject at

Note how WOTC has incorporated three into many elements of 4E. How many encounter and daily attack powers do you get? 3 (except PP powers). Your third encounter of a day is a milestone, and most typical sessions probably only take you to the third or MAYBE fourth encounter anyways (and if you go four, one of them was likely a cakewalk/quick). There are three tiers of play. If each session has three encounters and you throw in a quest or two, it takes about three sessions to level up.

Sticking to the rule of three keeps development manageable. It has a defined start, middle, and end. Progression is quick enough to be pyschologically satisfying.

Thus, I think the Rule of Three should be considered for use for elements that (A) should feel special or (B) are related to linear development (plot, session pacing, mechanical growth, etc). The mechanics of the system should help GMs use the Rule of Three to make more pyschologically sound scenarios.


Human memory seems to have a thing for 7 +/- 2 chunks of information. If we are presented with a choice of more than about seven options, we can't comprehend all of them at once. See Delta's article on the subject at

Note how WOTC has implemented this in some ways in 4E. A typical combat lasts about 7 +/- 2 rounds. A typical monster often goes down in 7 +/- 2 attacks.

When selecting powers, there are usually no more than seven choices per level (including MP).

Some places it is poorly implemented. For example, feats initially present far more than seven options. This requires the player to winnow down the list.

I find myself first eliminating feats I don't qualify for then chunking the remainder ones into offensive, defensive, and flexibility lists. I then decide which element I want to boost and then select from my sub-list. Equipment is another place it tends to be poorly implemented. There are far more than seven different types of viable magic armors or weapons which makes it tedious to hunt through all of them. I find that I don't have much patience any more for tracking GP or XP or even HP. I do it, but its tedious. I definitely need paper and pencil or a calculator. It is much easier to track AP or daily magic item usages remaining or even my stable of magic items (because I usually have 7 +/- 2 items!).

Magic Sevens should be used anywhere that a human is presented with a choice. It is also good for longer situations than the Rule of Three can handle. For example, one could either model combat on three 3-round "phases" or just use the Magic Sevens principle to steer combats towards seven rounds. The player should never be presented with a list of items longer than 7 +/- 2. Some exceptions might be made for round numbers like multiples of 10, as we are usually
quite familiar with the base 10 system. It might also be acceptable to use Imperial units of measure in some cases due to people's familiarity with them, even though they don't break down quite so nicely (12" to a foot, for example -- although many Imperial units are designed to be easy to use, such as a League!).


I like Imperial Units. On one hand they have an immediate weakness -- they're hard to convert. On the other hand, they're easy to use. Many are related to real-world experiences. For example, a league is about as far as you can walk in an hour or so. A stone is about as much weight as you can easily lift with a hand. A pint is what you drink at one sitting. Wherever they can be logically worked in, I think they are helpful.


Granularity refers to the amount of precision used in any mechanical check or decision point. See Delta's article at

This is one thing that 4E does not do well. Almost all checks are on a D20. This is good for consistency, but it implies that an out of combat streetwise check is as important as landing a life-or-death daily power. It forces the DM to call for rolls out of combat only when they are very significant, even though a DM might want to use dice to settle other situations. 4E is better than 3E in this regard because some powers and items allow auto-success; for example, for movement, many PCs will have a way to fly or teleport instead of relying on tedious skill checks.

One addendum for Delta's Article. Note that surprise was initially determined using D6s. As Gary's players reached the higher levels (D series instead of B2), surprise became more and more important. Notice how those later monsters (drow, spectators, duergar, etc) reduce the granularity of surprise checks from a D6 to a D8, D12, or even D100. As surprise became more important to life or death, it naturally deserved a more finely determined roll.

In general, I think one should use the smallest dice possible. One should use larger dice only when (A) the level of significance increases and (B) you intend to fully use the level of granularity provided (i.e., don't roll on a D20 but make all your modifiers or adjust target numbers in +3 chunks -- otherwise you might as well use a D6 and give a +1 modifier!).


A poll on Dragonsfoot awhile back indicated that many respondents wanted about a 2/3 chance to hit when swinging -- that is, when doing something they're good at. Note that in original 1E, a strong character opened a door on 3/6 or 4/6, a sneaky character surprised others 4/6 (elves, halflings, thieves, rangers taking precautions). 66% are good odds pyschologically. One can push the envelope up to 75% and still retain tension. Once the odds get up around the 90% point there's not much point in rolling -- that's the point when you are just seeing if a "1" comes up.


The system should use the simplest math possible, especially for frequently repeated or time critical operations.

The easiest operation is to compare two numbers and see what is greater. After that, addition is easiest. Subtraction is also pretty simple. Multiplication is more complicated, and division is hardest. Single digit numbers are easier to deal with than double digit numbers. The more numbers are involved the harder it gets ("Well, I have 1d8 weapon damage, +2d6 sneak attack, +4 DEX, +3 magic weapon... etc.). Precalculation can help with some of this.

4E is terrible with this. I can't imagine playing above heroic tier without use of the computer or a calculator right in front of me.


For a fantasy clone to feel like D&D to me, it needs certain elements. I guess these are "sacred cows."

- D20 rolls in combat.
- Fighters, Clerics, Wizards, and Thieves.
- Certain Races.
- Level-based system.
- Medieval weapons.
- Appropriate milleou.
- 3+1. By this, I mean for every 3 combat encounters, there should be 1 RP encounter/trick/trap/puzzle, in general. The original 1E is an outgrowth of

Chainmail, and later versions always reflect that wargame bias. A game like WoD is the opposte, 1+3 (1 combat encounter for every 3 RP/trick/trap/puzzle).

Obviously there are exceptions, but I think 3+1 works well for a typoical session worth of material.

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