Multi-Class and Dual-Class Characters (PHB)

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A multi-class character improves in two or more classes simultaneously. His experience is divided equally between each class. The available class combinations vary according to race. The character can use the abilities of both classes at any time, with only a few restrictions. Only demihumans can be multi-class characters.
A dual-class character is one who starts with a single class, advances to moderate level, and then changes to a second character class and starts over again. The character retains the benefits and abilities of the first class but never again earns experience for using them. There are some limitations on combining the abilities of the two classes but, as long as minimum ability and alignment requirements are met, there are no restrictions on the possible character class combinations. Only humans can be dual-class characters.
==Multi-Class Combinations==
All of the standard demihuman races are listed here, along with their allowable multi-class combinations. Note that the character class names (not group names) are used below.
Dwarf
* Fighter/Thief
* Fighter/Cleric
Elf
* Fighter/Mage
* Fighter/Thief
* Mage/Thief
* Fighter/Mage/Thief
Gnome
* Fighter/Cleric
* Fighter/Illusionist
* Fighter/Thief
* Cleric/Illusionist
* Cleric/Thief
* Illusionist/Thief
Halfling
* Fighter/Thief
Half-Elf
* Fighter/Cleric*
* Fighter/Thief
* Fighter/Mage
* Cleric*/Ranger
* Cleric*/Mage
* Thief/Mage
* Fighter/Mage/Cleric*
:* or Druid
As stated earlier in their description, specialist wizards cannot be multi-class (gnome illusionists are the single exception to this rule). The required devotion to their single field prevents specialist wizards from applying themselves to other classes. Priests of a specific mythos might be allowed as a multi-class option; this will depend on the nature of the mythos as determined by the DM.
==Multi-Class Benefits and Restrictions==
A multi-class character always uses the most favorable combat value and the best saving throw from his different classes.
The character's hit points are the average of all his Hit Dice rolls. When the character is first created, the player rolls hit points for each class separately, totals them up, then divides by the number of dice rolled (round fractions down). Any Constitution bonus is then added to the character's hit points. If one of the character's classes is fighter and he has a Constitution of 17 or 18, then he gains the +3 or +4 Constitution bonus available only to warriors (instead of the +2 maximum available to the other character classes).
Later the character is likely to gain levels in different classes at different times. When this happens, roll the appropriate Hit Die and divide the result by the number of classes the character has (round fractions down, but a Hit Die never yields less than 1 hit point). The character's Constitution bonus is split between his classes; thus, a fighter/mage gets of his Con bonus when he goes up a level as a fighter and the other of the Con bonus when he goes up a level as a mage. A fighter/mage/thief would get 1/3 of his bonus when he goes up as a fighter, 1/3 when he goes up as a mage, and the other 1/3 when he goes up as a thief.
If the optional proficiency system is used, the character starts with the largest number of proficiency slots of the different classes. Thereafter, he gains new proficiency slots at the fastest of the given rates. To determine the character's initial money, roll according to the most generous of the character's different classes.
:Rupert's character, Morrison the Multi-Faceted, is a half-elf fighter/mage/thief. At 1st level, Morrison rolls three dice for hit points: 1d10 (fighter), 1d6 (thief), and 1d4 (mage). The results are 6, 5, and 2. Their sum (13) is divided by three and rounded down to equal 4 (13/3=4-1/3=4). Morrison begins the game with 4 hit points. Later, Morrison reaches 2nd level as a thief before he reaches 2nd level as a fighter or a mage. He rolls 1d6 for additional hit points and the result is 4. He divides this by 3 (because he has three classes) and rounds down. Morrison gets 1 more hit point when he becomes a 2nd-level thief. (He will also roll 1d10 and 1d4 [both rolls divided by 3] when he reaches 2nd level as a fighter and as a mage, respectively.)
Multi-class characters can combine abilities from their different classes with the following restrictions:
Warrior: A multi-classed warrior can use all of his abilities without restriction. The warrior abilities form the base for other character classes.
Priest: Regardless of his other classes, a multi-classed priest must abide by the weapon restrictions of his mythos. Thus, a fighter/cleric can use only bludgeoning weapons (but he uses the warrior combat value). He retains all his normal priest abilities.
Wizard: A multi-classed wizard can freely combine the powers of the wizard with any other class allowed, although the wearing of armor is restricted. Elves wearing elven chain can cast spells in armor, as magic is part of the nature of elves. However, elven chain is extremely rare and can never be purchased. It must be given, found, or won.
Thief: A multi-classed thief cannot use any thieving abilities other than open locks or detect noise if he is wearing armor that is normally not allowed to thieves. He must remove his gauntlets to open locks and his helmet to detect noise.
==Dual-Class Benefits and Restrictions==
Only humans can be dual-classed characters. To be dual-classed, the character must have scores of 15 or more in the prime requisites of his first class and scores of 17 or more in the prime requisites of any classes he switches to. The character selects one class to begin his adventuring life. He can advance in this class as many levels as he desires before switching to another class; there is no cut-off point beyond which a character cannot switch. However, he must attain at least 2nd level in his current class before changing to another class. There is no limit to the number of classes a character can acquire, as long as he has the ability scores and wants to make the change. (Certain character classes have alignment restrictions that the character must meet, however.)
Any time after reaching 2nd level, a human character can enter a new character class, provided he has scores of 17 or better in the prime requisites of the new class. After switching to a new class, the character no longer earns experience points in his previous character class and he can no longer advance in level in that class. Nor can he switch back to his first class at a later date, hoping to resume his advancement where he left off. Once he leaves a class he has finished his studies in it. Instead, he starts over in a new class, at 1st level with 0 experience points, but he does retain his previous Hit Dice and hit points. He gains the abilities, and must abide by all of the restrictions, of the new class. He does not gain or lose any points on his ability scores (for example, an 18 Strength wizard who changes to fighter does not gain the percentile Strength bonus, but likewise a fighter changing to a wizard would not lose it). The character uses the combat and saving throw tables appropriate to his new 
class and level.
This is not to imply that a dual-class human forgets everything he knew before; he still has, at his fingertips, all the knowledge, abilities, and proficiencies of his old class. But if he uses any of his previous class's abilities during an encounter, he earns no experience for that encounter and only half experience for the adventure. The only values that can be carried over from the previous class without restriction are the character's Hit Dice and hit points. The character is penalized for using his old attack or saving throw numbers, weapons or armor that are now prohibited, and any special abilities of the old class that are not also abilities of the new class. (The character is trying to learn new ways to do things; by slipping back to his old methods, he has set back his learning in his new character class.)
In addition, the character earns no additional Hit Dice or hit points while advancing in his new class.
The restrictions in the previous two paragraphs last until the character reaches a higher level in his new class than his maximum level in any of his previous classes. At that point, both restrictions are dropped: the character gains the abilities of his previous classes without jeopardizing his experience points for the adventure, and he earns additional Hit Dice (those of his new class) and hit points for gaining experience levels in his new class.
Once these restrictions are lifted, the character must still abide by the restrictions of whichever class he is using at the moment. A dual-class fighter/mage, for example, cannot cast spells while wearing armor.
:Tarus Blood-heart begins his career as a cleric with a Wisdom of 16. He rises to 3rd level and then decides to become a fighter, since his Strength is 17. He keeps his 14 hit points (rolled on 3d8), but in all other ways he is treated as a 1st-level fighter. Upon reaching 4th level, Tarus is allowed to roll 1d10 for additional hit points. He can now cast spells as a 3rd-level cleric and fight as a 4th-level fighter. For the rest of his career, Tarus advances as a fighter but retains his minor clerical powers—a useful advantage when the situation gets ugly!
When a dual-class or multi-class character is struck by a level-draining creature, he first loses levels in the class in which he has advanced the highest. When his different classes are equal in level, the class level requiring the most experience points is lost first.
The player character is allowed to regain levels lost by level draining, but until he regains all of his former levels, he must select which class he will use prior to any particular adventure. Using abilities of the other class then subjects him to the experience penalties given earlier. When he regains all of his former levels, he is then free to use all the abilities of all his classes once again. Of course, he cannot raise his earlier class(es) above the level(s) he was at when he switched class.
:Tarus is a 4th-level cleric/3rd-level fighter. He is struck by a wight and loses one level from his cleric class, since it is his highest level. If struck again, he would lose one level from his fighter class. Thereafter he could regain his lost levels, but would have to choose to act as either a fighter or cleric. Once he earned enough experience to regain his previous fighter level, he would not be allowed to advance further in it (restoring himself to his previous level only). But he could still advance as a cleric and use his 3rd-level fighter abilities.