Captain Anorak's Guide to Gaming
Captain Anorak's First Rule for Game Mechanics:
A rule must be transparent

A rule in a game must be easily understandable and unambiguous. It must be completely clear: no interpretation must be needed by the GM or players to make the game playable. It must not encourage misinterpretation.

Transparency means that it is obvious what each component of a rule represents and how they interact with each other.

When you're GMing a roleplaying game, you sometimes find yourself with a situation that has to be represented in terms of game mechanics, but which is not covered in the rulebook. You then have to make decisions about whether and how to adapt the rules to cover this situation (see here).

If you have to make these decisions, it's much easier if the rules are transparent, ie. it's easy to see by looking at them what parts of the game mechanics system correspond to what elements of reality. So, when writing rules that someone else will have to run, it's a good principle to always make them as transparent as possible.

EXAMPLE: ROLLING TO HIT

This is an example of poor transparency that I've run up against time and time again in games like Cthulhu and Runequest (the system now called Basic Roleplaying) and others such as Paranoia and Last Unicorn's Star Trek (see also here). Such games have a simple roll to hit. So a fairly competent character might have a 40% chance to hit. A defending character then has a chance to defend by parrying or dodging, again by a percentile roll.

This might represent combat tolerably well as long as both combatants are mobile, up and fighting. But what happens when the situation is changed? Suppose I want my character (weapon skill 40%) to hit an opponent who is lying unconscious on the ground, while there is no combat going on around. In terms of the reality represented, I should be able to do this with no or virtually no chance of failure. But some GMs with whom I've played these games use the following rules: I still have to roll to hit as normal, but the target gets no defence. This is plainly ludicrous in terms of what would happen in reality, but the GM has looked at the rules and thought 'The hit roll represents the chance of hitting and the defence roll represents the chance of defending; the target can't defend so I'll keep the attack roll the same but not allow a defence roll.'

The rulebook could remedy this very simply by adding a single line along these lines: 'Attacking an unoving target (such as an unconscious person or an inanimate object) gives the attacker a +100% bonus to hit'. This would not make the rules more bulky or cumbersome, but would increase the realism, and make all GMs use the same system for attacking unmoving targets, instead of relying on each GM to come up with his own interpretation of the rules. But the writers of these games don't even seem to have considered this possibility. They haven't thought about any combat situation other than the normal one. This makes them bad writers.

Apologists for the CoC/Runequest combat system have tried to justify it in these terms: the hit chance represents the chance of hitting someone under the normal conditions of combat. This means that the attacker and target are both moving about, and that attacks are imprecise due to the excitement of combat.

My own experience with martial arts tells me that this is untrue. I've done a lot of sparring, and what I've seen is that people simply don't miss with the kind of simple, basic techniques which combatants use most of the time. With straightforward meat-and-potatoes moves like punches and kicks, no able-bodied adult ever misses - not even a complete beginner. Unless the target deflects or dodges the blow, it will hit.

A simple and transparent system could work like this:

A character has a Combat Skill with the following values: Crippled Poor Average Good Heroic 0 4 8 12 16 and a Dodge skill with the following values: Immobile Sluggish Average Lithe 0 7 14 21 A fighting character splits his Combat Skill into Attack and Parry. To hit an opponent, a combatant must roll 1D20 and add his Attack, and get a result above the target's (Parry + Dodge). Thus if two average characters are fighting, and both choose not to parry at all, simply relying on their dodging skill to defend them, then they both have Attack 8, Parry 0, Dodge 14. This means that to hit the other, one combatant needs to roll over 14 one 1D20+8, ie. he needs a natural 7+ (a 70% chance of hitting). If they both decide to split their Combat Skill into 4 Attack and 4 Parry, then they both have Attack 4, Parry 4 and Dodge 14. To hit the other, one combatant needs to roll over 18 one 1D20+4, ie. he needs a natural 15+ (a 30% chance of hitting). An immobilised character gets Dodge and Parry 0. A very slow-moving slug-monster would have Dodge 0 (or very low) but if it had limbs it would still be able to parry. A character not in the heat of battle gets a +4 bonus to hit. This gives an increased chance of hitting to a character who is not panicking or rushing around. A character fighting multiple opponents must split his Dodge between them, and must split his Combat Skill up into separate Attack and Parry values against each opponent.

The system described makes everything transparent. It's obvious what Attack, Parry and Dodge represent. It's obvious that since an unmoving character can't dodge, he gets Dodge 0. This doesn't need any thinking about. The system give the combatant some choice over how much effort to put into attack and how much into defence, and it's clear exactly how this works. So this is a highly transparent system.