Multiclassing
Multiclassing is when a player chooses to gain experience points and train in multiple classes simultaneously. There are two types of multiclassing. The first form of multiclassing is switching, and the second form is dualing.
Switching is the default method. The player must already have a class, whether it is level one or higher, before attempting a switch. To switch a class, the player must first select a new class, and will then start at level 1 (or level 0 if applicable, see magic users and knights). When the player switches, they will cease to gain experience for their previous class. Additionally, they will only gain experience if performing feats, tasks, and combat tactics used by the new class selected by the player. For example: If the player starts out a fighter, and chooses to go into magic-user, the player will only gain experience in their new class if they engage in combat primarily using spells (as well as any method that's useful) with fighter skills and tactics being used as a minor.
Dualing is the secondary method and while more beneficial in the long run, it is more difficult. With this method the player either starts out with, or switches to (using the above method) leveling two levels simultaneously. As this is a unique leveling method, there are a few minor rule changes. For one, the player will not gain experience points unless utilizing both class abilities/tactics in sessions. When experience is given to the player dualing classes, the experience points will be split as evenly as possible between the two classes. Thus while leveling is twice as long and difficult, they will receive large bursts of ability, skill and power due to the two classes. When leveling up the player will roll HP using each class's HD (for example, a thief/illusionist would roll 1d6 and 1d4) and add each die's number to their max hp.
As a rule, all races can only have a maximum of two classes (except for humans, who can have up to three classes).