BTW, what is classic multi- and dual-classing?
Multi-classing, available only to characters who are not human, involves a character who is a Fighter/Magic-User (or several other combinations depending on race/edition) at 1st level. They gain experience as normal, but that experience is split between the two classes. So where a Fighter would require 2000xp to reach 2nd level, a F/MU will only reach 2nd as a Fighter when they have 4000xp overall (and 5000xp to reach 2nd as MU). Mostly you get the combined abilities of both classes, so the F/MU can train any weapons that a Fighter could train, fights like a Fighter, has the better saving throws of the classes involved; one exception being hit points, where you get half as many from each class when it goes up (one third as many if you're triple-classed), and another being that if a class was banned from using certain items then it didn't matter that the other was permitted them, so a F/MU usually didn't wear armour.
Dual classing was available only to human characters. Start off normally with one class, then you had the option (I think between 3rd and 9th level only) to drop the first class and advance in a second. Until your second class exceeded the level of your first, you couldn't use any abilities from the first without losing all your experience from that adventure. On the other hand, you retained the hit points and I think saving throws. And once you reached a higher level with your second class, you could use the first classes abilities freely. So if your character started as a Thief, reached 3rd level and restarted as a Fighter, they couldn't use their thievery skills or backstab until they were a 4th level Fighter, but once they did they were free to carry on.