Can anybody clear up the difference between these two. I know how they are different ie, dual you change to a new class and cannot go back, multiclass you start as eg fighter/mage - whats the advantage of dual over multi. If I start again would a good choice be kensai/mage multi class right from the start. do I get the fighter bonus for hitpoints?

My head spins

Only humans can dual-class and they can't mulitclass .
In Dualclass you choose a new "proffession" and then you start gaining levels in that calss instead. You will never be able to go back to your old calss but you will be able to use the skills in the first class when your level of the second class is as high as the first.

I think a Kensai/mage should be dualclassed at 9th or 12th level.
A multiclassed character can't specialise into, for example kensai, but have to be a clean fighter/?

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"No!" shreiked Tyrion. Though the sword felt heavy as a fallen tree he lifted it. He saw the deamon look up, astonished. He brought down the blade with the awesome majestic power of a falling thunderbolt. The burning blade caught the deamon right in the centre of its forehead. The deamon's head fell apart under the force of the impact. The deamon N'kari sank to its knees, dead.

-Tyrion at the battle of the Blighted Isle.
Right now he is mourning for USA.

A further difference becomes obvius at high levels (if ToB is installed or if XP cap has been removed): the multiclass character will be able to obtain the special "feats" from BOTH classes (example: greater whirlwinds from fighter AND use any item of thief), while the dual class character can only get feats from the class he has dualled into (fighter-->thief could only get feats from the thief list, would not have access to the whirlwinds/critical hits/etc).

Also, if you plan to use a Kensai/mage (with ToB installed or XP cap removed) you should probably dual class at level 12 to maximize the Kensai abilities. With ToB installed, this will only lessen the mage's maximum level by 1, so you will still get nearly all of the skills/spells available to the mage (don't have the book in front of me to determine whether you lose one 9th level spell due to this or not).

--Greystone

You lose one 8th level spell if you dual a Kensai to Mage at level 12.