Your character has six ability scores:
In normal humans, these six ability scores will range in value from 1-20, with the average being 11 and almost everyone falling in a 3-18 range. Ability scores have an absolute maximum score of twice the human maximum, i.e. 40.
Ability Score Generation
When creating a character, your ability scores are given the following innate values in a random order: 17, 16, 14, 12, 11, 10. These innate base values are then each given modifiers for your class and your race to form your actual starting ability scores.
Innate Scores and Actual Scores
It is possible for your innate ability scores to be increased or reduced by the use of magic. The most common sources of such magic are potions and mushrooms. Innate ability scores have an absolute minimum of 3 and an absolute maximum of 28, and magical modifications will not put them outside this range. Items that increase your innate ability scores will have a lesser effect when your innate scores are higher than the normal human maximum of 20, and may only increase a score by a fractional amount. Such fractional scores are not shown on the character sheet, but are tracked internally.
However, you will have other bonuses and penalties to your ability scores that can change the actual scores, making them not match the innate scores. Every race (other than human) and every class has a set of bonuses and/or penalties to ability scores; and many mutations and magic items can also alter your actual ability scores. In the case of race and class adjustments, these are permanent, but the adjustments due to mutations and items are more temporary in nature – mutations can be lost, and items only provide their adjustment while they are being wielded or worn.
Note that if an unidentified item is wielded, it will still have its normal effect on your ability scores and this effect will be shown on your character sheet even though the item has been not been identified. This can be a good indicator that the item is more special that it might first appear.
The minimum for actual ability scores is 3, and the maximum is 40. Actual scores will not go above or below these limits.
Ability Score Damage
Ability scores can take temporary damage from a variety of sources. The most common source of ability damage is as a side-effect of normal damage, for example acid can leave you scarred or fire can leave you weakened. Some monsters have special attacks that specifically damage your ability scores in this way, as do some traps. Such damage temporarily reduces your actual score, and it can be healed by magical means. Some races are protected from having particular ability scores damaged, and you can also acquire items that will protect your ability scores.