Humans can normally operate only in one class at a time. However, in rare circumstances, a human may leave behind their original class and begin anew in another class. This is a complex procedure with strict prerequisites and ongoing consequences. In most cases, it is simply better to start a new character from scratch. If chosen, the character will strive to focus in their new class over continuing to use abilities in their original class. Dual-class is not chosen lightly and is not meant to function as does multi-class characters.
You must be human (and not already dual-class or have taken a level in an advanced class). Exception. Half-elf single-class fighters under 8th level may dual-class as a thief if they have declared an intent to become a bard.
You must be at least 3rd level in your original class.
You must have a 15+ in all of the Primary Ability Scores of the original class or an 18+ in a single primary ability score of the original class.
You must have a 16+ in all of the Primary Ability Scores of the new class.
A character who’s original class is a paladin or monk will lose the abilities of these classes if they dual-class (your original class becomes a fighter (paladin) or thief (monk) of your current level).
A character who’s original class is cleric should have good reason (according to their deity! per the GM) for abandoning their faith—otherwise, your original class will become a fighter of 1 level less than your original class level.
The new class cannot be a sub-class of the original class or a class that does not begin at 1st level.
The character immediately ceases all function of the original class and drops (forfeits) experience points to the beginning of the original class’ current level. (e.g. A fighter attained 3rd level plus another 1,200 XP. They train to level 3 and then forfeit any additional XP, in this case 1,200).
The character will then begin the training necessary to become 1st level in the new class. This will take at least 1 week per class level of the original class (per the GM) and will cost 1,500 gp per week in addition to any required equipment (spell books, holy symbols, thieves tools, etc.). The mentor doing the training must dedicate full time to the student's training and must be at least 6th level in the class the student is training toward. In this case, the mentor can be another PC, although the expense remains the same.
Once the training period is complete, the character will become, in many respects, a 1st level character of the new class.
If a character’s new class is a fighter, ranger, or paladin and the PC has an 18 Strength, that character may roll for Exceptional Strength.
The character is granted new initial Weapon Proficiencies suited to the new class.
Level or Hit Dice. With regards to effects that require knowing a creature’s level or hit dice, your character uses the sum of all hit dice (where hit points were provided by the level, see Hit Points below). Energy drain affects the new class levels first (without a loss in hit points if no hit points were gained).
Experience. All experience going forward will apply to the new class.
Attacks. Your character will use the best AAC0 available to either class.
Saving Throws. Your character will use the best saving throw available by category available to either class.
Hit Points. You stop gaining hit points for each level until your new class becomes higher level than your old class. At that point, you can roll hit points for your new class normally. Your Constitution bonus applies to each hit die normally (i.e. if you were previously a fighter with an 18 Constitution, you don’t lose any hit points but if becoming a non-fighter the maximum Constitution bonus for hit points for new hit dice is +2).
Weapons Allowed. While the dual-class character is lower level in the new class than the original class, the character will prefer to use the new class' weapons over that of the original class (assuming they are different). The character can continue to use weapons not allowed by the new class (the proficiency is not forgotten), but if this occurs, even in a single combat, all XP gained for the encounter* will be worth half of the normal awarded XP. Once the character is equal to or greater in level in the new class versus the original class, this preference/restriction goes away. In addition, weapon restrictions are subject to class function rules (as appropriate—a back stab attack must be performed with a piercing weapon for example).
Armor Allowed. In order to perform functions of the new class, the appropriate armor (including none) must be worn according to the new class.
Spell Casting and Magic Abilities. The dual class character will prefer to operate as the new class over that of the original class, especially while the new class level is lower than that of the original class. If the original class granted the ability to cast spells or magical abilities, the character can use these, but all XP gained for the encounter* will be worth half of the normal awarded XP. Once the character is equal to or greater in level in the new class versus the original class, this preference/restriction goes away.
Starting Wealth. When switching to the new class, no starting wealth is gained.
Restrictions or Strictures. You must abide by any restrictions or strictures for all classes or the appropriate consequences will accrue. These are listed in the class statistics under alignment restrictions or additional restrictions. If any are in conflict (e.g. Ranger and Assassin alignment restrictions), you will lose access to the original class (or it will change based on the class description). If losing access to the new class within the first three levels, you forfeit all experience points gained by that class (becoming 1st level again) unless atonement is a possible remedy.
* Encounter Definition. While the definition of an "encounter" is per the GM, it is typically defined as a "major encounter area". ie. A full level of an extensive dungeon complex, a castle/keep, or a set of encounters during a long trip.
When beginning a player character higher level than 1st level, dual-class characters must be specifically approved by the GM. Even if approved, an experience point expense will typically be assigned (e.g. 60% of the XP available to other starting characters). Magic item awards would not be altered (ie. assignments would be the same for a dual-class character as for those beginning at a higher level.). The XP adjustment is meant to take into account a character using all granted abilities at times, even against preference/focus.