Yet another thing to add Lol - Multi-Classing & Dual-Classing!
Multi-Classing is an option that was only available only to Non-Human characters in AD&D, its the simultaneous advancement in two or more different Classes. The Basic D&D Elf Class was really a Multi-Classed Fighter/Wizard.
Dual-Classing, which was an option only available only to Human Characters, is the ability to switch classes and begin Advancing exclusively in a new character class. Sort of like changing careers.
There is specific information about the interaction of the restrictions and abilities of Multi-Classed Characters. Gnomish Multi-Classed characters are (for example) limited to Leather Armour in order to cast spells, while Elven Multi-Classed Characters are not. These restrictions reflect the particular nature of the non-human races.
It is a general rule however, that a Cleric/Fighter may always use edged weapons.
When a non-human character has more than one character class, any experience points gained by the character will be divided evenly between these two classes, even once the character can no longer progress in one of the classes.
When the character advances in one of his or her classes, additional hit points are determined by rolling the appropriate die (D4 for Wizards and Illusionists, D6 for Thieves and Assassins, etc.), adding the appropriate constitution modifier, and dividing by the number of Classes involved (so two for a Cleric/Thief, or three for a Fighter/Cleric/Wizard, for example).
It is therefore entirely possible for a multi-class character to be at different levels of experience in different Classes. As an example, a Half-Orcish Cleric/Assassin of Level 10/12 is possible.
In order to switch from one class to another, the human character must have an ability score of 15+ in the Classes Prime Attribute of the original Class, and of 17+ in the Prime Attribute of the new Class.
The character retains his or her Hit Points.
He or she can use the particular abilities of the original class, but doing so will cause the character to forfeit all experience points that would be gained from using the original Classes Abilities (as determined by the GM). This restriction applies until the character has advanced in the new Class to a Level at least one higher than was attained in the original Class.
When the character begins the process of learning a new class, he or she gains the Abilities of the new Class as a First Level member of that Class, and all Experience is applied to progressing in the new class.
The Character gains no additional Hit Points until reaching a Level in the new Class one Level higher than in the original Class, after which time the Character’s Hit Points advance as per the new class.
Unlike multi-class characters, dual-classed characters cannot perform the functions of several different classes simultaneously. So, for example, an Elven Fighter/Wizard could cast Arcane Spells while wearing Armour, but a Human Dual-Classed Fighter/Wizard would have to remove his or her Armour to do the same thing.
However, with dual-classed characters, any combination of classes is possible, provided the character qualifies in terms of Primary Ability Scores and can comply with any relevant Alignment restrictions. An individual with very high Ability Scores could therefore play a Cleric/Paladin or a Wizard/Illusionist.