Multiclassing in 3.5 can be pretty complicated. 1) If the classes are not within one level of each other you get an experience penalty. Note: your favored class does not count against this. 2) Skills: Maximum ranks are equal to your character level+3 (sum of all levels) if one of your classes has that skill as a class skill. If none of your classes have that skill as a class skill maximum ranks are equal to (character level+3)/2. When spending ranks you spend them depending on the class you are leveling. Spending ranks in a class skill is 1rank = 1rank. Spending ranks in a cross-class skill is 1rank = 1/2rank. For example: Lets say you are a Fighter/Rogue and you just gained a level in fighter. You want to put two fighter skill ranks in stealth. You put 2 in and only get 1 rank. Next time you level you level up as a rogue, you spend 1 rank and get 1 rank in stealth. This is because stealth is a cross-class skill for fighters and a class skill for rogues. 3) Base Attack Bonuses: Add up all the bonuses from all the classes. Example: A Fighter 5/Rogue4 would have a Base Attack Bonus of +5 and +3 for a total of +8/+3. 4) Base Save bonuses: Add up all the bonuses from all the classes. Example: A Fighter5/Rogue4 would have a Base Fort of +4 and +1 for a total Base Fort of +5. 5) Ability advancement (such as spellcasting). You do not advance in a class' abilities unless the class specifically says you do. Example: You are a Fighter4/Rogue4 and you go to Fighter5/Rogue4. You do not gain the level 5 rogue abilities such as an extra d6 of sneak attack damage. Final note: I havent played 3.5 since moving to Pathfinder, there might be other elements of multiclassing I am missing. Sidenote, Dual Classing is a 2nd edition term. They did away with it in 3.X/PF. - Gauss
