|d20 Modern System Reference Document|
Every character has six basic Ability Scores:
The Score of these Abilities ranges from 0 to infinity. A limit, if any, will be specified in the rules. The normal human range is 3 to 18. It is possible for a creature to have a score of "none". A score of "none" is not the same as a score of "0". A score of "none" means that the creature does not possess the ability at all. The modifier for a score of "none" is +0.
A character with a CON of 0 is dead. A 0 in any other score means the character is helpless and cannot move.
Keeping track of negative ability score points is never necessary. A character's ability score can't drop below 0.
Each ability will have a modifier. The modifier can be calculated using this formula: (ability/2) - 5 [round result down]
The modifier is the number you add to or subtract from the die roll when your character tries to do something related to that ability. A positive modifier is called a bonus, and a negative modifier is called a penalty.
Table: Ability Modifiers
The ability scores help define your character. The 3 physical ability scores are Str, Dex and Con. The 3 mental ability scores are Int, Wis and Cha.
Any creature that can physically manipulate other objects has at least 1 point of Strength.
Any creature that can move has at least 1 point of Dexterity.
If a character's Constitution changes enough to alter his or her Constitution modifier, his or her hit points also increase or decrease accordingly at the same time.
Any creature that can think, learn, or remember has at least 1 point of Intelligence. A creature with no Intelligence score is an automaton, operating on simple instincts or programmed instructions. It is immune to all mind-influencing effects (charms, compulsions, phantasms, patterns and morale effects) and automatically fails Intelligence checks.
Any creature that can perceive its environment in any fashion has at least 1 point of Wisdom.
Any creature capable of telling the difference between itself and things that are not itself has at least 1 point of Charisma.
Changing Ability Scores
Ability scores can increase with no limit.
Poisons, diseases, and other effects can cause temporary ability damage. Ability points lost to damage return naturally, typically at a rate of 1 point per day for each affected ability.