D20 System: used in D&D 3.x and variants, this at it's basic core is a roll of d20+modifier vs. a Difficulty Class. Rolling equal or above the DC is a success.
This is a good mechanic. I've touched it up a bit.
Modos RPG: roll d20 + ability modifier + skill, versus an opposed roll.
- If the opposition is just difficulty, then the GM rolls d20 + difficulty.
- Equal rolls are ties. Ties can be rerolled, or treated as ties.
AD&D 2ed: attack rolls. To determine if an attack scores damage, an attacker needs a THAC0 and the defender needs an AC.
- Determining THAC0: class levels award THAC0, which stands for To Hit Armor Class 0 (zero).
- Armor and dexterity
lower Armor Class.
If a character needs X To Hit Armor Class 0, then he needs X-Y To Hit Armor Class Y. Which is to say, roll X-Y or higher to hit armor class Y.
Simple, right? I've touched it up a bit.
Modos RPG: attack rolls. To hit your opponent, your attack roll (d20 + ability mod + skill) must exceed your opponent's defense roll (d20 + ability mod + skill). Simple. And directly related to the core mechanic above.
Dragon Age: attack rolls. Roll 3d6 plus strength plus focus. Beat the target's defense to hit. But if you rolled doubles, you get the number on one of the double-dice to spend in stunts. Stunts are, well, like critical hits, but more interesting.
Dark Heresy: nope. I won't even try. Because in this system, your success in attacking is based on your own ability, not the enemy's. But then the enemy gets a chance to defend anyway, not based on your roll, but on his ability. Or, um, I don't know. But the Inquisition is coming...