The Core Mechanic

The Core Mechanic of the game is that whenever your success is in question you

• Roll 1d20
• Add the modifiers for the situation (such as an attack bonus or a skill bonus)
• Then see if you get equal to or above the situation's Difficulty Class (DC)

If you do, your attempt succeeds, if not your attempt fails.

That's it, that's like most of the rules right there. The rest of the game is just stuff that tells you what numbers you get to add to your d20 roll in what different situations.

• If you don't know how to resolve a situation, assume that it's some sort of skill check.
• If no skill on the skill list seems to apply, then it's probably an ability check.
• If none of the ability scores seem to apply then you're in a very rare situation indeed. At that point just roll a d20 and everyone in the group can decide as a group if it was a good enough result or not.

Multiplying

When you multiply game values (like an attack or damage bonus) only the biggest multiplier counts at full value. Additional multipliers count as x1 less before adding them to the biggest multiplier. For example, if one effect caused x3 damage and another caused x2 damage, the total damage would be x4.

Another way to think about it is that x2 is really "+100%", and x3 is really "+200%", so when you combine them you get "+300%", which is x4.

Non-game values (like height, weight, and time) use normal math. If a spell makes a person twice as big (giving x8 weight) and then another spell turns them stone (giving about x3 weight) then their giant statue form weighs 24 times as much as when they started.

Rounding

Unless specified otherwise, always round down after a division. Despite this being the rule, many things will remind you to round down anyway. Oh well.