In d20 games that do not feature classes, skills are the bread and butter of modeling a character concept. The following rules introduce an added level of flexibility to skills. Without classes, characters should be able to advance in a more organic manner, with their in-game experiences helping to mold their capabilities. For example, in fantasy d20 a fighter lives for battle. He approaches an adventure ready to engage the enemy in combat and takes the forefront in any conflict. In contrast to this definition of a character's role through his class, characters in class-less d20 games may mold their skills and talents in response to the dangers they face, particularly in genres where the characters begin play knowing little of the dangers they face. A character could potentially begin play as a mild-mannered department store stock boy and eventually transform into a chainsaw-wielding, shotgun-toting champion of humanity who routinely puts down powerful undead demons.
Gaining New Class Skills
Over the course of a character's career, he may decide to pick up a new skill through study, experience, or hard work. A character's selection of class skills reflect his training before embarking on his career as an investigator but they do not reflect the knowledge and expertise he acquires over the course of his career. For example, a physics professor may learn of magic, retire from research, and devote his waking hours to uncovering arcane theories. While such a character may not have begun play with Spellcraft as a class skill, it makes sense that he may eventually treat that skill as a class one.
You may gain a new class skill at every level evenly divisible by 3. At each such level, you may spend 4 skill ranks to make a cross-class a class one. If you previously purchased skill ranks in a newly minted class skill, you may now treat those ranks as having been spent on a class skill. For example, a character who treats Spot as a cross-class skill has spent 6 ranks on it, granting him a total bonus due to skill ranks of +3. If he spends the skill ranks to change Spot into a class skill, he now gains a +6 bonus to Spot checks from those skill ranks.
If you do not select a new class skill at a level divisible by 3, you may "save" that new class skill for later. Just because you do not have a class skill in mind does not mean you lose out on the option to take a new one. However, you must wait until you increase in level before you select the new class skill.
You may only change a cross-class skill into a class one if you have spent at least 6 skill ranks on it. Gaining a new class skill is the end result of hard work and study. You cannot suddenly develop a strong interest in an area of study you previously ignored. When you spend ranks to gain a new class skill you may not spend ranks to improve that skill. For example, at 3rd level Jack spends 4 skill ranks to make Gather Information a class skill. He may not spend any remaining ranks he has to improve his Gather Information skill. By the same token, he could not spend the ranks on Gather Information and then purchase it as a class skill. He must do one or the other.
Author's Note: I'd like feedback on the paragraph above. Do you think it's fair to require a character to train in a skill before making it a class one? On one hand, that's realistic, but on the other it forces players to sit on their hands for a level if they want to expand in a completely new direction. Send email to mearls AT alum.dartmouth.org
You do not have to purchase a new class skill when you have the ability to do so. This is a purely voluntary character option.
Areas of Expertise
Some people pour so much time and effort into a skill that they are better at using it than their associates with similar backgrounds and experience. At 1st-level, you may opt to purchase a skill expertise associated with a single class skill. An area represents a specific subset of the talents covered by a skill. When you use a skill under the conditions or area of knowledge associated with your expertise, you gain a +2 competence bonus to your skill check. You may purchase one new skill expertise at 5th, 10th, 15th, and 20th-levels. You may gain each specific expertise once. However, you may purchase more than one expertise for a skill. For example, at 1st-level you could gain Hide Expertise (rural) and at 5th-level purchase Hide Expertise (urban).
The following list summarizes the available skill expertise options.
(List forthcoming when the author has time to poke his head out from underneath a half-dozen game design projects! Please be patient!)