As anarchistica said, you ALWAYS get exp penalties (and they're pretty harsh!) if you multiclass unless:
1. The class is your favoured class
or
2. The classes are maximum one level from each other.
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EXAMPLES:
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EXAMPLE 1: A Half-Orc
1-1. You have a half-orc Barbarian(8)
-It's only one class, so there are no penalties.
1-2. You have a half-orc Barbarian(8)/Fighter(1)
-Barbarian is the half-orc's favoured class, so it doesn't count. Thus, it's like you had just a half-orc fighter(1)
1-3. You have a half-orc Barbarian(8)/Fighter(1)/rogue(1)
-Barbarian is the half-orc's favoured class, so it doesn't count. Thus, it's like you had just a half-orc fighter(1)/rogue(1).
-The fighter and rogue levels are within 1 level of each other, so it doesn't give an exp penalty.
1-4. You have a half-orc Barbarian(8)/Fighter(4)/rogue(1)
-Barbarian is the half-orc's favoured class, so it doesn't count. Thus, it's like you had just a half-orc fighter(1)/rogue(1).
-The fighter and rogue levels aren't within 1 level of each other, so it gives an exp penalty.
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EXAMPLE 2: A Human
[This one is done as if you're leveling it up]
1-1. You have a human cleric(1)
-It's only one class, so there are no penalties.
1-2. You have a human cleric(1)/druid(1)
-The cleric and druid levels are within 1 level of each other, so it doesn't give an exp penalty.
1-3. You have a human cleric(2)/druid(1)
-The cleric and druid levels are within 1 level of each other, so it doesn't give an exp penalty.
1-4. You have a human cleric(3)/druid(1)
-Cleric level is highest, thus making it the human's favoured class. So, it doesn't count! Thus, it's like you had just a human druid(1)
1-5. You have a human cleric(3)/druid(1)/monk(1)
-Cleric level is highest, thus making it the human's favoured class. So, it doesn't count! Thus, it's like you had just a human druid(1)/monk(1)
-The druid and monk levels are within 1 level of each other, so it doesn't give an exp penalty.
1-6. You have a human cleric(4)/druid(1)/monk(1)
-Cleric level is highest, thus making it the human's favoured class. So, it doesn't count! Thus, it's like you had just a human druid(1)/monk(1)
-The druid and monk levels are within 1 level of each other, so it doesn't give an exp penalty.
1-7. You have a human cleric(4)/druid(2)/monk(1)
-Cleric level is highest, thus making it the human's favoured class. So, it doesn't count! Thus, it's like you had just a human druid(2)/monk(1)
-The druid and monk levels are within 1 level of each other, so it doesn't give an exp penalty.
1-8. You have a human cleric(4)/druid(3)/monk(1)
-Cleric level is highest, thus making it the human's favoured class. So, it doesn't count! Thus, it's like you had just a human druid(3)/monk(1)
-The druid and monk levels aren't within 1 level of each other, so it does give an exp penalty.
1-9. You have a human cleric(4)/druid(3)/monk(2)
-Cleric level is highest, thus making it the human's favoured class. So, it doesn't count! Thus, it's like you had just a human druid(3)/monk(2)
-The druid and monk levels are within 1 level of each other, so it doesn't give an exp penalty.
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...You probably get the idea by now! It doesn't need to fit both requirements, only one