If you buy a class skill, your character gets 1 rank (equal to a +1 bonus on checks with that skill) for each skill point. If you buy other classes’ skills (cross-class skills), you get ½ rank per skill point.
Your maximum rank in a class skill is your character level + 3.
Your maximum rank in a cross-class skill is one-half of this number (do not round up or down).
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.
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.
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, among others.
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 ½ 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 ½ 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.