Your maximum rank in a skill is your character level.
To make a skill check, roll: 1d20 + skill modifier (Skill modifier = skill rank + ability modifier + miscellaneous modifiers)
This roll works just like an attack roll or a saving throw – the higher the roll, the better. Either you’re trying to match or exceed a certain Difficulty Class (DC), or you’re trying to beat another character’s check result. For instance, to sneak quietly past a guard, an infiltrator needs to beat the guard’s Perception check result with her own Move Silently check result.
Note that normally a natural 20 is a critical success and gives one step higher result than normally calculated where such matters and a +5 from the calculated value in skill versus skill contests.
A character’s number of ranks in a skill is based on how many skill points a character has invested in it. Many skills can be used even if the character has no ranks in them; doing this is called making an untrained skill check.
The maximum ranks you can have in a skill is your level.
It is possible to specialize in a sub skill instead of the full skill. By using one rank you can get +2 to a single sub specialization of a skill instead of a normal skill rank. There is no limit in the number of sub specialization bonuses you can have, but there is no point in learning more than one specialization of any single skill if main skill is available.
The specializations do not count towards skill ranks in the skill. This means that they do not count for things that require a given number of ranks in a skill and do not count towards the maximum ranks in the skill.
Profession is a special case where there is no main skill, instead you always learn specializations.
Note that most NPCs have most of their skill ranks in specializations.
The ability modifier used in a skill check is the modifier for the skill’s key ability (the ability associated with the skill’s use). The key ability of each skill is noted in its description.
Miscellaneous modifiers include racial bonuses, armor check penalties and bonuses provided by feats, favorable or unfavorable conditions (this bonus is granted by the GM, if he so decides it), among others.
Some checks are made against a Difficulty Class (DC). The DC is a number set by the GM (using the skill rules as a guideline) that you must score as a result on your skill check in order to succeed.
A skill check represents an attempt to accomplish some goal, usually while under some sort of time pressure or distraction. Sometimes, though, a character can use a skill under more favorable conditions and eliminate the luck factor. Taking 10: When your character is not being threatened or distracted, you may choose to take 10. Instead of rolling 1d20 for the skill check, calculate your result as if you had rolled a 10. Distractions or threats (such as combat) make it impossible for a character to take 10.
Taking 20: When you have plenty of time, you are faced with no threats or distractions and the skill being attempted carries no penalties for failure, you can take 20. In other words, eventually you will get a 20 on 1d20 if you roll enough times. Instead of rolling 1d20 for the skill check, just calculate your result as if you had rolled a 20.
Taking 20 means you are trying until you get it right and it assumes that you fail many times before succeeding. Taking 20 takes twenty times as long as making a single check would take.
Since taking 20 assumes that the character will fail many times before succeeding, if you take 20 on a skill that carries penalties for failure (for instance, disarming a bomb), your character would automatically incur those penalties before completing the task (in this case, the bomb would most likely set off). If take 20 is possible, so is take 10.
Ability Checks: The normal take 10 and take 20 rules apply for ability checks.
You can help another character achieve success on a skill check by making the same kind of skill check in a cooperative effort.To do so, you must make a check for the intended skill against a DC of half that of the main users DC(or half the opponents success). If you succeed, you provide a +2 bonus to your ally’s skill check.
Multiple allies can make an Aid Another but the GM might impose limitations.
Aid Another cannot be used for social skills and the Sense Motive skill unless all characters are actively interacting and talking.
Aid Another usually requires coordinating and talking about things, so normally requires things like the ability to communicate, common language and such. Though simple tasks might be coordinated by gestures and the like.
If you succeed your check, the character you are helping gets a +2 bonus to his or her check (you can’t take 10 on a skill check to aid another). In many cases, a character’s help won’t be beneficial, or only a limited number of characters can help at once. The GM limits cooperation as he or she sees fit for the given conditions.
In cases where the skill restricts who can achieve certain results(like rank limits), you can’t aid another to grant a bonus to a task that your character couldn’t achieve alone.
Long tasks with aid another tend to take longer, due to the need to coordinate, especially with large numbers of helpers.
Sometimes a character tries to do something to which no specific skill really applies. In these cases, you make an ability check. An ability check is a roll of 1d20 plus the appropriate ability modifier. Essentially, you’re making an untrained skill check. The GM assigns a Difficulty Class, or sets up an opposed check when two characters are engaged in a contest using one ability score or another.
In some cases (especially those with prolonged use of a specific ability score) an action is a straight test of one’s ability with no luck involved. Just as you wouldn’t make a height check to see who is taller, you don’t make a Strength check to see who is stronger.