If a multiclass character's classes are not nearly the same level, then he suffers a multiclass penalty of -20% XP for each class that is not within 1 level of his most experienced class. These penalties apply from the moment the character adds a class or raises a class's level too high. For instance, a 4th-level wizard/3rd-level rogue gets no penalty, but if that character raises his wizard level to 5th, then he would receive the -20% penalty from that point on until his levels were nearly even again. The penalty is -40% if all three levels are uneven, for instance, a 5th-level wizard/3rd-level rogue/1st-level fighter.
Races and multiclass XP Edit
A racially favored class does not count against the character for purposes of the -20% XP penalty. For instance, an 11th-level gnome character (a 9th-level rogue/2nd-level wizard) suffers no XP penalty, because he has only one non-favored class (wizard is favored for gnomes). Suppose he then achieves 12th level and adds a level of fighter to his classes, becoming a 9th level rogue/2nd-level wizard/1st-level fighter. He suffers a -20% XP penalty on future XP he earns, because his fighter level is so much lower than his rogue level.
A dwarf 7th-level fighter/2nd-level cleric suffers no penalty, nor does he when he adds 1st-level rogue to his classes since his cleric and rogue classes are only 1 level apart, and fighter is a favored class for dwarves.
Human is not always the best choice to avoid an XP penalty. For example, a human 2nd-level fighter/8th-level wizard/8th-level rogue would suffer a -20% XP-penalty, a dwarf with the same class combination would not. Humans and half-elves can never suffer a -40% penalty.
Because polymorphs (including spells like shapechange and a shifter's abilities) change a creature's race, the multiclass penalty can also change while one is in effect. Generally, excluding some rare cases in which the opposite can hold, this means sometimes introducing a penalty for Tenser's transformation and the drow form of humanoid shape, while reducing or eliminating the penalty for other polymorphs. More precisely, the polymorphed character uses the favored class of the new race, and the favored class of all non-playable races is the same as for humans (the highest base class). Furthermore, the standard polymorphs that change a creature to a playable race are only available to players via Tenser's transformation and the drow form (both change the race to "elf").
Prestige classes and multiclass XP Edit
Prestige classes are exempt from the multiclass XP rule and do not count against the character. For example a 20th-level character (10th-level rogue/3rd-level shadowdancer/7th-level assassin) will not suffer any XP penalties no matter what race the character has chosen.