Like all d20 games, the Ground Zero rules use a core mechanic for all actions, keeping play fast and intuitive. This involves the use of a twenty-sided die (or 'd20'). Whenever the GM or the rules require you to see if your character succeeds with a task (such as an attack, the use of a skill or ability, or an attempt to save yourself from harm), you do this:
If the roll equals, or is higher than, the target number, your character succeeds. Otherwise, he fails. The d20 governs a range of rolls that you can think of generally as 'checks'. You 'check' whether you hit in melee, whether you sneak past the security bot, whether you figure out how to use an Ancient device, and so on. While the die used is the same, however, it gets applied in a variety of ways.
The d20 rules use dice to answer two basic questions: 'whether' and 'how much'. While the 'whether' dice is almost always the same (the tried and true d20), the 'how much' dice vary (d4, d6, d8, d10, and d12). For ease of reference, the rules abbreviate dice rolls with phrases such as '3d6+1', which means roll three six sided dice and add one to the result (generating a number between 4 and 19). The first number tells you how many dice to roll (you add them all together), the number after the 'd' tells you what type of dice to use, and the expression after that indicates a quantity you add, subtract, multiply, or divide.
In addition to the common d20, the 'd%' is occassionaly used to answer the question 'whether' when the outcome is truly random. When d% is specified, a number between 1 and 100 is generated by rolling two different ten-sided dice. One die (designated before you roll) is the tens digit. The other is the ones digit. The percentile format is a reminder that nothing modifies the roll. Percentile rolls are used for mutant perks and flaws, random encounters, scavenging results, and miss chances due to concealment or blindness.
Important: Not every action requires a die roll. Roll dice in combat an other dramatic situations when the success of an action is in doubt.
In general, if you wind up with a fraction, round down, even if the fraction is one-half or larger. For example, if a grenade blast deals you 13 points of damage, but you succeed at your saving throw and only take half damage, you take 6 points of damage.
Exception: Certain rolls, such as damage, have a minimum of 1.
Sometimes a special rule makes you multiply a number or a die roll. As long as you’re applying a single multiplier, multiply the number normally. When two or more multipliers apply, however, combine them into a single multiplier, with each extra multiple adding 1 less than its value to the first multiplier. Thus, a double (×2) and a double (×2) applied to the same number results in a triple (×3, because 2+1=3).
Every bonus you get a d20 roll represents a way in which your character is above average, and every penalty represents a way in which he is below average. A character who is completely average, therefore, has no bonuses or penalties. This concept is the 'stock human standard'. If the Game Master has to make a roll for some random NPC, he can simply roll the d20, add nothing, and use the roll as it is. The NPC tries to trick the characters? The GM rolls d20 to see how well he pulls it off. He tries to punch a PC? The GM rolls d20 to see how good his left jab is. The NPC tries to resist a character’s psionic mind control? The GM rolls d20 for the saving throw.
The rule for the average bonus, of course, applies only to NPCs of the 'average' genotype - stock humans. Mutant characters, even average ones of their type, have bonuses and penalties. Anyone with the 'furry' genotype receives a +2 bonus to Constitution, but have a -2 penalty to Intelligence. A run-of-the-mill furry gets a +1 bonus to Fortitude saves to resist the detrimental effects of radiation, but has a -1 penalty on Use Technology checks to figure out Ancient relics. Other genotypes also have bonuses and penalties representing how they differ from the stock human average.