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 nonhuman 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 fi ghter 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.