|System Reference Document v3.5|
Essentially, this system treats each skill as either a known skill or an unknown skill. A character is considered to have the maximum number of ranks allowable in any skill known to that character, and no ranks in any skill unknown to that character.
Each character begins play knowing a number of skills equal to the number of skill points his character would gain with each new class level. When creating a character, choose a number of skills known based on your first class level (see Table: Skills by Class). Human characters add one to their number of known skills.
For classes or monster types not listed on the table, simply assign a number of skills known equal to the number of skill points gained at each new level or Hit Dice, plus the character's or monsters intelligence modifier.
Note whether each skill known is a class skill or cross-class skill for your character, based on his starting class.Table: Skills By Class
For example, Fnord the gnome bard has an Intelligence of 12 (+1 bonus) and thus may know seven skills at 1st level. He selects the following seven skills (and denotes Forgery as a cross-class skill): Perform (string instruments), Spellcraft, Use Magic Device, Gather Information, Listen, Decipher Script, and Forgery (cc)
As with a normal skill check, you also add any other modifiers that apply (racial bonuses, ability modifiers, bonuses from synergy, and so forth).
For example, at 5th level, Fnord has a Charisma of 16 and a masterwork lute. When he makes a Perform (string instruments) check, he uses this formula:
When he makes a Forgery check, the formula changes to reflect the fact that Forgery is a cross-class skill:
Skills that don't have ranks, such as Speak Language, work differently. A character who chooses Speak Language as a known skill learns four new languages at 1st level and one new language at each level thereafter (if Speak Language is a class skill) or two new languages at 1st level and one new language at each odd-numbered level thereafter (if Speak Language is a cross-class skill).
For example, to use the song of freedom bardic music ability, Fnord must be at least 12th level and have at least 15 ranks in a Perform skill. Since Perform is a class skill for bards, and Fnord has selected Perform (string instruments) as one of his skills, he is treated as having the maximum possible ranks in Perform (string instruments). Thus, by 12th level, he is considered to have 15 ranks in Perform (string instruments). At the same time, he is considered to have only 7 ranks in Forgery (a cross-class skill).
First, compare your current list of known skills to your new class's list of class skills. If any of your new class's class skills are currently known by you as cross-class skills, you may choose one such skill and designate it as a class skill. This change immediately increases your modifier for the chosen skill (see Skill Modifiers, above). Every time you gain a level in this new class, you may choose another such skill, if applicable, and change it from a cross-class skill to a class skill.
Second, compare your new class's number of skills known at 1st level to the number of skills known 1st level from the class you already have. If you already have more than one class, compare it to the class that would grant the highest number of skills, even if that wasn't your first class. For example, if you are playing a 2nd-level rogue/5th-level fighter and pick up a level of barbarian, use the rogue's number of skills (8), even though you have more fighter levels than rogue levels.
If your new class grants an equal or lower number of skills, you can choose one skill from your new class's list to add to the skills you know as a class skill. You immediately gain the new skill at the maximum possible tank.
If your new class grants you up to four more skills than your best previous class, you may choose two skills from your new class's list to add to the skills you know as class skills. You immediately gain the two new skills at the maximum possible rank.
If your new class grants you five or more skills over and above your best class, you may choose three skills from your new class's list to add to the skills you know as class skills. You immediately gain the three new skills at the maximum possible rank.
Perform this operation every time you gain 1st level in a new class.
This system reflects the idea that by multiclassing, you learn more abilities and, in particular, skills. Most characters do end up with higher numbers of skills and ranks if they multiclass under this variant system, but they still have fewer skill choices.
If you increase your Intelligence modifier, you learn one additional skill. Make this skill choice using the class skill list of any of your existing classes. You can choose a class skill or a cross-class skill.
Alternatively, your game master can allow the Skill Knowledge feat, which increases your characters number of known skills.
When a character makes a skill check under this variant skill system, use these formulas:
Here, the trade-off for simplicity is ability. Characters with a large number of class skills are extremely versatile, but even their average skill check result is lower than that made by a character using the existing skill system. For skills outside their areas of expertise (cross-class skills), they have nothing beyond their own basic abilities and perhaps magic or other bonuses.
Under this system, all barbarians are good at climbing, jumping, listening, riding, and the like, but have virtually no ability to bluff, hide, search, or perform other cross-class tasks. Monks become superior acrobats (with talent in balancing, climbing, jumping, and tumbling) but no proficiency at anything outside the idiom of the typical monk (such as disguise or survival).
A character who wishes to specialize or branch out her skill knowledge can still select feats such as Skill Focus, Alertness, and the like, regardless of whether the skills affected by the feat are class skills or cross-class skills. This is the best avenue for a fighter who wishes to improve his awareness, a wizard who wants to become intimidating, and so on.
Skills that don't have ranks, such as Speak Language, work differently. A character who has Speak Language as a class skill learns four new languages at 1st level and one new language at each level thereafter. Characters without Speak Language as a class skill can't learn new languages without selecting the Skill Knowledge feat and designating Speak Language as a class skill.Skill Knowledge feat (see below) can be selected by a character to add to his number of skills known. In this case, the feat only allows you to designate a cross-class skill as a class skill (since you already know all your class skills).
Benefit: Choose any two skills from one of your current classes' skill lists. You now know these skills as class skills.
Special: Instead of choosing two class skills, you may choose one cross-class skill (whether you already know it or not) and learn it, treating it as a class skill from that point forward.