Some characters are exceptional characters, to the point where they can pursue more than one class at a time. The act of doing this is called multi-classing.
They progress through the selected classes, but much more slowly than a character who has dedicated themselves to just one class.
Multi-classing is far more complex than single classed characters. If you are a beginner to RPGs, (or heck, even bad at math) you may not wish to play multi-class characters. However, they also can be fun, allowing characters you might not otherwise be able to play. If you wish to try to multi-class, read on.
You can Multi-Class into 2 or 3 classes, but never more than three.
A 2-Class Multi-Class is read as: Class(Level)\Class(Level)
A 3-Class Multi-Class is read as: Class(Level)\Class(Level)\Class(Level)
Your race will tell you which Multi-Class combinations are valid for a character.
For many characters, your Race will tell you what Multi-classes you can be. However, in addition, there are some ground rules for multi-classing for races such as human or half-elf.
In each classes gameplay section, you will find the class’s Archetype. You can never multi-class into classes which contain the same Archetype. Finally, some classes have multiple archetypes, so keep an eye on them.
Multi-class characters have to abide by all rules of any class they multi-class into. For example, a Fighter can use light, medium, and heavy armor, but a Magic-User can only cast spells while wearing no armor Thus, a Fighter\Magic-User can wear light, medium, or heavy armor, but cannot cast spells if they are wearing any.
In order to enter into classes, you must meet all normal ability score prerequisites for all classes, if there are any. However, due to experience point penalties that multi-classing grants you, it is only recommended (but not required) that you multi-class if you are able to off-set the experience point penalties with experience point bonuses offered by having higher ability scores.
Some races have additional multi-class rules. For example, gnomes cannot multi-class into any class with the priest archetype.
Some classes have additional multi-class rules, which will be listed in the gameplay section for the class.