SRD:Skills Summary

Skills Summary
Each skill point you spend on a class skill gets you 1 rank in that skill. Class skills are the skills found on your character’s class skill list. Each skill point you spend on a cross-class skill gets your character 1/2 rank in that skill. Cross-class skills are skills not found on your character’s class skill list. (Half ranks do not improve your skill check, but two 1/2 ranks make 1 rank.) You can’t save skill points to spend later.

The maximum rank in a class skill is the character’s level + 3. If it’s a cross-class skill, the maximum rank is half of that number (do not round up or down).

Regardless of whether a skill is purchased as a class skill or a cross-class skill, if it is a class skill for any of your classes, your maximum rank equals your total character level + 3.

Using Skills
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.

Epic Skill Uses
Despite the near-magical nature of some epic level skill uses, all uses of skills are considered exceptional abilities (except if noted otherwise), and thus function normally even within areas of antimagic.

Skill Ranks
A character’s number of ranks in a skill is based on how many skill points a character has invested in a skill. Many skills can be used even if the character has no ranks in them; doing this is called making an untrained skill check.

Ability Modifier
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
Miscellaneous modifiers include racial bonuses, armor check penalties, and bonuses provided by feats, among others.