[whswhs]

As far as Common Sense goes:

There are tasks you might need to perform that call for some skill. For example, you might use Survival to identify which mushrooms are edible, or Savoir-Faire to negotiate some issue of court protocol. If you don't have the skill, your chances are poor: Per-5 or IQ-4, in these cases. If you critically fail, something harmful or seriously costly happens.

There are tasks where IQ would be sufficient, but if you have a certain skill, you could use that instead, if it's higher. These are easier and less dependent on specialized knowledge. For example, you might use IQ or Savoir-Faire to figure out which fork to use.

There are tasks that default to the skill at a bonus equal to the usual default penalty—so you can roll vs. the unmodified stat, OR you can roll vs., say, Survival+5 or Savoir-Faire+4. So maybe you're trying to gather firewood, and Per will let you spot dry wood, but Survival+5 will boost your odds because you know where to look. And maybe a critical failure gets you green wood, or wood that gives off mildly toxic smoke and contaminates your food.

Even easier than those are tasks that don't normally call for a skill roll AT ALL. Any normal person can just say, "Okay, I say 'Good morning. Can I help you?'" or "I get out my matches and light the fire." You can talk about GMs calling for rolls to do them, but (a) they would need bigger bonuses still, because IQ alone should be enough to succeed on 16 or less, let's say, and trained skill would get you even higher, and (b) really, asking for a roll in that case is a bit of an exercise in sadism, unless you're trying to do slapstick comedy. And if you fail at that roll, you feel incredibly inept; if you critically fail, you have an embarrassing accident that actually hurts you in some minor way.

But because players are perverse, you can have a player character who intentionally does something that is as bad a mistake as a critical failure on one of those last rolls, and that is going to have destructive consequences. And you might want to say, "So you want to pour lighter fluid onto your campfire so it will burn hotter?" or, "So you spit on the Duchess?" The consequences may well be bad enough so that you wouldn't go that far even after a standard critical failure; they're outside the realm of ordinary random bad luck and into that of "what could they have been thinking?"

And those last actions are what Common Sense is meant to prevent. Where a normal player wouldn't do that, and a perverse or stupid player would do it, such a player with Common Sense would be told, "Roll your IQ," and if they succeeded, the GM would say, "That's the kind of thing that will give you large area burns/land you in the ducal dungeons."

This might be adapted to making up for the game world not being like the mundane world. But really I think mistakes of that sort are better dealt with by the GM just telling the player that it's a bad idea because X. I think Common Sense is better used for players who habitually do dumb ****. You know, like throwing a grenade into the starport office because their permission to take off is being held up.

Now, I put this in terms of asking the player to roll IQ, because I have players make their own rolls. If your style is to roll things for players, you might want to roll this. It's still a roll vs. IQ, whether the player or the GM makes it.

But because players don't normally fail at tasks as simply as politely greeting the Duchess, or make mistakes as bad as having their character spit on her, and because a GM wouldn't normally have anything so awful happen, or make the player roll anything to avoid it, there is no roll to modify by IQ bonuses. Common Sense gives you an IQ roll, if the GM decides you need it. So the bonus only applies if you already have Common Sense.