SYSTEM REFERENCE DOCUMENT 3.5
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 1/2 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).
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.
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.
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.