Characters in YAGS will normally have a skill in the range of 0 to 10. Talents are common skills which everybody has at least some ability in - they are things like Athletics, Charm and Stealth. All characters start with a skill of 2 in these.
4 is the minimum level of professional skill, and most professionals will have a skill in the range of 4-5 in their chosen subject. This is a common starting level for new characters.
Skills of 8 to 10 are considered the upper range for most people, and it is generally recommended that 'unnamed' NPCs won't have a skill higher than 10. These people are the masters of their profession.
The very best people in the world can have skills in the range of 12 to 15. The lower end of the range may be the best in a fantasy setting, the higher end in a modern or futuristic setting where there is often much more opportunity to learn and study a specialist subject. However, there is no real upper limit. If someone is immortal, or extremely long lived, higher skill levels may be achievable.
When using a skill, it is multiplied by the character's relevant attribute. The average adult human has a score of 3 in all their attributes. Since a d20 is added to this, a typical professional will be rolling (3 x 4) = 12 + 1d20.
Typical attribute upper limits for characters are in the range of 5-6. This means masters of their profession can potentially be getting a skill bonus of 60 to 100.
The Target Number (TN) is the number you need to roll in order to succeed at something. If you roll that number or higher, you succeed. For every 10 points you gain an extra level of success, which may be beneficial in some circumstances. Some example TNs are as follows.
10 - A very easy task that a professional can achieve automatically.
20 - A moderate task that a professional has a 65% chance of achieving, but which less skilled people will struggle to succeed at.
30 - A hard task that most professionals will struggle with (15% chance of success)
Some TNs can reach 100 or more, but these are reserved for seemingly impossible tasks which most experts will need to find ways to reduce before they can tackle them.
Getting a good balance between too few skills and too many skills can be quite hard, especially when the definition of these limits varies from player to player.
Unless there's a good reason, YAGS tries to keep similar skills which would realistically be all known by a particular type of character as a single skill. For example, Brawl covers punching, kicking, wrestling and dodging - few characters would be interested in learning only one of these and not others, and it's fairly realistic to bunch all such abilities together. Exotic martial arts also come off Brawl - anyone specialised in such things is going to be good at Brawl as well, so it makes sense for them to be a specialisation of the broader skill.
In fact, all combat skills are heavily simplified. There is a single Melee skill which covers all melee weapons not covered by Brawl. Throw covers rocks, spears and grenades. Guns covers all forms of personal firearms.
Likewise, Science is a single skill which covers everything from particle physics to genetics. Obviously, this is a broad range of subjects and it's unrealistic to have a single skill cover everything. This is where Techniques come in.
Broad skills also greatly simplifies how related skills are dealt with. For example, if each martial art was it's own skill, how do we handle a street brawler with a Brawl of 6 who starts learning Karate? If they have a Karate of 1, how do they compare to someone with a Brawl of 2 but a Karate of 3? Either the lower skill is never used, the skilled fighter is greatly disadvantaged if he tries to use new skills, or we need a complex system of skill relationships.
Some examples of skills are given on the following pages:
A technique is like a specialisation that can be taken with a skill. When a skill reaches a certain level, a technique can be purchased which either mitigates certain penalties, or allows the character to do something they wouldn't otherwise be able to do.
For example, Rifle allows a character to fire all sorts of rifles in all sorts of ways. When their skill reaches level 2, they can purchase the Quick load technique which enables them to reload a magazine as a free action. Other techniques reduce accuracy penalties when firing automatic bursts, or give greater bonuses when aiming.
Knowledge skills, especially academic ones, may have a number of specialisations associated with them. These are implemented as techniques. Unlike techniques which allow a character to perform a special action (such as combat techniques), a specialisation does not need to be known in order to attempt a task that falls within one, however they do make the task easier.
The difficulty of knowledge based tasks (such as Science) is based on how rare the subject is, and how much detail is required. World War 2 is a fairly common subject for the purposes of the History skill (from a Western, early 21st century perspective). Being able to describe the uniforms worn by the major forces during the same period requires detailed knowledge which would make the knowledge check harder. Recognising who Stephen of Blois was, and knowing when he was King of England and details of his civil war with Empress Matilda would be a much rarer subject, so even basic information would be difficult to know with basic History.
When assigning a difficulty for a knowledge task, it is necessary to determine how rare the knowledge is, and how detailed the information being requested is. This will depend heavily on the specialisations known. As a start though, the following rough guidelines can be used for difficulty levels.
Common information which pretty much anyone who has studied the subject will know, and which people with a passing knowledge have a good chance to know. Even someone with 1 or 2 in the skill are likely to succeed, and those defaulting from a related skill of 4 or more have a good chance to succeed.
Uncommon information which only students of the subject are likely to know, but which is fairly common knowledge amongst such students. A skill of 4 gives a 65% chance of success, assuming average intelligence. Defaulting to this skill requires a roll of 40+, which is very difficult.
Detailed or rare information which only a specialist is likely to know. You need a high intelligence or a 6+ skill to start having a reasonable chance of success.
Very rare or obscure knowledge.
Specialisations are generally kept generic - it is up to the player and GM to decide what are suitable. For example, there is a single Historian (specialist) technique which can be taken multiple times with different specialisations. The specialisations could be Kings of England or Saxon England. Both would make knowledge of Hardicanute common, but the first would also provide knowledge of Henry VIII, and the second knowledge of the Dane Geld. It's very much up to the player how they chose to specialise.
Some skills are a bit more codified. Science has three defined specialisations, each of which has a more detailed (open) specialisation based off it.
How difficult it is to know an item of information will depend on the techniques that a character knows. For example, knowledge that atoms are made of protons, neutrons and electrons may be a very easy check using Science. Knowing the fundamental forces (weak, strong, electromagnetic and gravity) may be moderate difficulty. However, it's fairly common Physics knowledge, so would be very easy to someone who had that technique.
If a character wanted to build a nuclear bomb (and had the materials at hand) using Science, then it would be very rare knowledge (it's definitely part of Physics (uncommon), speciality nuclear physics (rare) specliased in nuclear weapons (very rare). She would need detailed knowledge of how to construct the bomb as well, making it around difficulty 60. If they have nuclear physics then it drops to about 30, if they have nuclear weapons then 20. Such an undertaking would be more complex than a single roll, but the general difficulty of her side of the project would be around 20.
The other type of techniques are Familiarities. These are similar to specialisations but tend to be used in practical skills rather than knowledges. Familiarities tend to be well defined for a particular skill. For example Drive will have a familiarity for car, lorry, bike, ATV, HGV, tank. Not having the familiarity counts as a default, halving your roll for all skill attempts.
Skills which require familiarity will have a default familiarity which is obtained automatically as part of the skill. For example, Drive assumes familiarity with cars. If you want to be good with a motorbike, you need to buy this as an extra. It's not perfect (since YAGS doesn't allow you to be good at riding a motorbike, but not a car), but it keeps things consistent.
One of the features that Techniques allow is to extend the skill system in a simple way to allow more cinematic game play. It is suggested that the real difference between run of the mill experienced people and true heroes is that the latter tend to have far more techniques. Whilst you may gain a high skill simply from doing the same thing for several decades, techniques are only learned through dedicated training.
The core YAGS rules allow for some quite powerful techniques, but they are alll grounded in reality. There is no reason why a set of martial arts techniques couldn't be added in that allow characters to act like cinematic heroes, for example a chain of Gun Kata techniques which are completely unrealistic, but which may fit well into a Pulp or Anime style campaign.
Currently, such techniques aren't in the pipeline, though they are definitely an option for a later supplement.