Captain Anorak's Guide to Gaming
Running Games with no Rules
Almost all commercially produced roleplaying games have a game mechanics system which is used to describe action that happens. This may perhaps be necessary in commercial games which are written for a mass audience (though I'm not convinced that it is necessary), but if you're writing a game just for your own circle of players then I would seriously think about whether rules are needed or not.

There are several related questions here: (1) should there be game mechanics or not; (2) if yes, should these game mechanics be known to the players; (3) if the characters have statistics to define their abilities, should these be known to the players or not?

HOW DO YOU RUN WITHOUT RULES?

I've run dozens of scenarios without game mechanics. I have verbal descriptions of the characters' abilities. When someone tries to do an action, I either decide what his chance of success is, or I simply decide whether he succeeds or fails.

THE FEELING OF DEFINED REALITY

When you have game mechanics, there is a certain feeling of solid ground beneath your feet. There are rules in the book which determine the chance of success. These are the same every time, so there's no arbitrary decision-making. It's 'fair' in that every character gets a chance of success dependent just on his stats; two characters with equal stats should always have the same chance of success when attempting the same action. However, in any sensible game system the GM will have to make some kind of decision about how difficult to do a thing is, so in fact there will be an arbitrary judgement there which will affect the final chance of success.

KNOWING HOW GOOD YOU ARE

In reality people don't know exactly how likely they are to succeed at a certain task. Having the ability to precisely work it out is unrealistic. In reality, you often don't know if you can do something until you try it. Worse, when players know their charcters' stats they often compare them to see who is best at a certain thing. This would not be possible in reality; furthermore, it removes an important tension from the game caused by not knowing who is the best. In cinema, there are often climactic points in the film where two characters go up against each other to settle once and for all who is the best. This can't happen in roleplaying if players can see each others' stats.

One advantage of seeing a defined chance of success is that it does give you a feeling of how difficult something is. This gives players a feeling of risk which they might get from doing something risky in reality, but which they don't get sitting around a table.

EXAMPLE: This is from The King's Men, a game which I once ran with a rules system that I knew but the players didn't. I made all the dice rolls myself, in secret. In the course of a battle on horseback, a mounted charcter took a spear in the guts. I made a roll for him to stay alive. If memory serves me correctly, he had to roll, on D100, Constitution x3 (=36) or less to stay alive and Constitution x1 (=12) or less to stay conscious. I rolled and remarkably he stayed conscious. He then had to roll to avoid falling off his horse. I think the roll needed was Ride skill at a -40 penalty, which came to 20%. Again I rolled and he made it. So his chance of staying conscious and on his horse was 2.4% and he made that by extreme good luck. I tried to impress on people how lucky he'd been but I didn't convince them and they went away saying 'This game's really unrealistic. He should have just died.' Of course he should; his chance of instant death was 64%. If the players had seen these numbers they would have realised just how incredibly spawny he'd been instead of slagging off my game.

Another advantage of defined rule systems is that different people have different ideas about how the world works and without numbers to refer to, then the GM just has to make a decision based on his own ideas, which may well be flawed. Having said that, a rules system can only cover a certain number of possibilities, and in practice most of the situations which a GM will have to judge on will not be covered.

GAME EMPHASIS

One of the most important arguments to me is about the emphasis of the game: what the players are thinking about when they play. To my mind, a roleplaying game is about playing characters, and the only purpose of any game mechanics system is as a tool to represent the reality of the game world. The rules should not be a prominent part of the game which the players are thinking about all the time; this distracts them from their proper business of roleplaying their characters. Thus if there is no game system known to the players (whether there are no rules at all, or whether the rules are known only to the GM), this distraction is kept away from them and so they concentrate on roleplaying not on rules.