A character’s class is a matter for the creating player’s choice, within the range of classes for which the character qualifies by virtue of his or her ability scores and race. Some classes — particularly those with very high ability score requirements, such as paladin — should be very rare within the campaign, and even if some exceptional player character qualifies for the class, non-player characters of that class will be very uncommon.
Non-player characters will, for the most part, lack any kind of class at all. Such individuals are called “0-level” and their abilities are subsumed into fighters, hence the “0-level” entries for fighters; all player character fighters should begin at level 1.
Characters will rise in level as they gain XP (experience points).
XP are gained by achieving goals within the game. The most frequent way will be defeating foes, but XP may also be awarded for non-combat oriented achievements as well.
Characters with especially high characteristic values may get a bonus to the XP they are awarded. The relevant characteristics are listed under each individual class, where they exist at all.
Such characters get a +10% bonus on awarded XP.
Note that some classes require several characteristics to be above certain levels, while others require that at least one of several characteristics be high enough. In the latter case, a character with more than one of the required stats above the required level still only gets +10% — you don't get a bonus for each stat!
Multi-classed characters may assign bonuses for high characteristics only to the XP awarded for the class for which that stat is relevant.
Treasure can be used as XP too, but only when formally training to rise in level, and only up to 50% of the XP required for the new level.
For example: A level 5 Fighter is training for Level 6.
L5 is achieved at 16,000 XP, L6 is achieved at 35,000 XP, so the fighter needs a total of 19,000 XP to get from L5 to L6.
Up to half of those XP (9,500) could be offset by spending gold instead, as training costs.
Formal training to rise in level is only absolutely necessary if the character does not have sufficient XP to ascend to their next level by "on-the-job" experience. However, there may be some additional formalities (and expenses) required in some circumstances, such as for example, having one's new rank recognised by a Thieves' Guild or a College of Magic, or some similar organisation. Wizards will only be given new spells when they train; otherwise they will have to obtain them by other means.
It is almost inevitable that some players will be tempted by what they see as the vast profits to be made out of training others instead of going out into danger themselves. They should rapidly become aware of the very high costs involved in running any training operation — hiring or buying premises, licence fees, the cost of obtaining sparring partners or exotic creatures or substances.... at the very best, a character who tries this pusillanimous tactic should receive maybe 5% of the nominal training cost. (I would suggest 1d10-5%, so from month to month they might make anything from a 4% loss to a maximum of a 5% profit). Not to mention that setting up in competition with other training organisations is not going to engender harmony amongst the community.
I discard dual-classing; everyone can multi-class, as long as they have the minimum racial and characteristic requirements.
You can start a new class when training to go up a level, but from then on all XP are split between all your classes — there'll be none of this taking one level in a class to get its benefits and then ignoring it forevermore.
Hit-points are generated exactly the same way as for single-classed characters; the hit-die will of course depend on whichever particular class is advancing.
Experience splitting between classes continues only until a class level limit is reached.
Example: a halfling is limited to level 4 as a fighter, but is unlimited as a thief. Once a halfling fighter-thief reaches 4th level as a fighter, and can no longer advance in that class, they then start devoting all their experience points to advancing as a thief.
Multi-classing, which is an option available only to non-human characters, is simultaneous advancement in two or more different classes. Dual-classing, an option available only to human characters, is the ability to switch classes and begin advancing exclusively in a new character class. Specific information about the interaction of the restrictions and abilities of a multi-classed character are described in the racial description of each race. Gnomish multi-classed characters are, for example, limited to leather armour in order to cast spells, while elven multi-classed characters are not. These restrictions reflect the particular nature of the non-human races and thus are detailed in the race descriptions. There is a general rule, however, that a cleric/fighter may use edged weapons. When a non-human character has more than one character class, any experience points gained by the character will be divided evenly between these two classes, even once the character can no longer progress in one of the classes.
When the character advances in one of his or her classes, additional hit points are determined by rolling the appropriate die (d4 for magic users and illusionists, d6 for thieves and assassins, etc.), adding the appropriate constitution modifier, and dividing by the number of classes involved (so two for a cleric/thief, or three for a fighter/cleric/magic user, for example).
It is therefore entirely possible for a multi-class character to be at different levels of experience in different classes. As an extreme example, a half-orcish cleric/assassin of level 4/15 is possible.
Optional Rule: Some GMs limit elven and half-elven fighter/magic users and fighter/magic user/clerics in respect of the armour they may wear while casting spells. If this optional rule is in play, such a character may only cast arcane spells if unarmoured — unless they are wearing elfin chain. The character may still cast divine spells regardless of the armour he or she is wearing. (This rule assumes that elfin chain is specially designed to allow fighter/magic users to wear it.)
Dual-class characters are humans who elect to change irrevocably from one class to another. The character class of a non-human represents talents and abilities that are somewhat more innate than those of humankind. Humans are more flexible and can generally become more skilled in any one class than non-humans, but lacking the innate talents of the other races, they can only focus upon one class at a time.
In order to switch from one class to another, the human character must have an ability score of 15+ in the prime requisite(s) of the original class, and of 17+ in the prime requisite(s) of the new class. The character retains his or her hit points. He or she can use the particular abilities of the original class, but doing so will cause the character to forfeit all experience points from that adventure (as determined by the GM). This restriction applies until the character has advanced in the new class to a level at least one higher than was attained in the original class.
When the character begins the process of learning a new class, he or she gains the abilities of the new class as a first level member of that class, and all experience is applied to progressing in the new class. The character gains no additional hit points until reaching a level in the new class one higher than in the original class, after which time the character’s hit points advance as per the new class.
Unlike multi-class characters, dual-classed characters cannot perform the functions of several different classes simultaneously. So, for example, an elven fighter/magic user could cast arcane spells while wearing armour, but a human dual-classed fighter and magic user would have to remove his or her armour to do the same thing.
However, with dual-classed characters, any combination of classes is possible, provided the character qualifies in terms of ability scores and can comply with any relevant alignment restrictions. Theoretically, an individual with very high ability scores could play a cleric/paladin or a magic user/illusionist.