Dice Rolls
RPGStuck uses different sorts of dice (or dicebots), the d20 chief among them. The d20 is a twenty-sided funny-looking polyhedron made usually of plastic or metal, with the occasional die made of wood, ivory, or other exotic materials. Often, you will end up using a dicebot, which automatically rolls a number for you, using the power of technology to always give you, and only you, the worst rolls at the worst possible times.
The d20 is used in attack rolls and skill checks, the two main types of rolls by which uncertainty is resolved in this system. Whenever an uncertain action is taken, a roll is made and compared against some threshold: if the roll matches or beats this threshold, the roll is considered a success. The SM then determines what happens, based on the result.
Attack Roll
The attack roll is made when its namesake is attempted: a hammer swung at an Imp, firing a gun at an Ogre, psionic eye lasers at the Black King. An attack roll rolls a d20, then adds Proficiency, stat modifier, and any relevant bonuses/maluses. The result is compared against the target’s relevant defense, which represents their ability to shrug off or avoid harm. The defenses are armor class, fortitude, reflex, and will, and are specific to the target.
The skill check is made when some challenging task is attempted: climbing a mountain, fixing a bike, finding Waldo. A skill check rolls a d20, then adds skill points, stat modifier, and relevant bonuses/maluses. The result is often compared to a difficulty class, or DC, a measure of the difficulty of the task.
Diceplay
There can be a variety of circumstances and forces at work that help or hinder your rolls. These are represented by various bonuses and maluses, or anything that modifies the roll that isn’t Proficiency, skill points, or stat modifier. They either come as flat numbers (such as firing into cover, or aiming for their leg) or as additional dice (such as Aim, Block, Stakes, and Flaws dice).
There is also advantage and disadvantage, which represent fairly hefty circumstances and forces that work in your favor or against you. When rolling at advantage, you roll a second d20 and take the higher d20 roll. Similarly, when rolling at disadvantage, you roll a second d20 but take the lower d20 roll. The two cancel each other out; if you have both, you roll as if you had neither.
However, they can stack in certain situations: if you would have advantage from two or more sources and you have nothing that imposes disadvantage, you additionally take +4 to the roll or check. Similarly, if you must have disadvantage from two or more sources and you have nothing that gives advantage, you additionally take -4 to the roll. In either case, if you have both, they all cancel out and you roll as if you had neither, no matter how many instances of each you have.
Rounding
If you’re ever dealing with dice and you end up with decimals, say you had to halve a roll, you typically round down, unless the reason that makes you have to round in the first place tells you to round up, then you round up.