multi-class and dual-class

Demihumans can be multiclassed characters while Human characters can change classes during their career making them dual-classed.

multiclasses

Multiclassed characters are demihumans who choose more than one profession. While this allows a greater range of skills, advancement is slower because experience is divided equally among classes. Not every demihuman race can choose all available multiclasses so refer to the Races page for a listing of available classes by race. Available multiclasses are: Fighter/Thief, Cleric/Mage, Cleric/Thief, Fighter/Druid, Fighter/Mage, Mage/Thief, Fighter/Cleric, Ranger/Cleric, Fighter/Mage/Cleric, and Fighter/Mage/Thief.

Multiclass characters can combine abilities from their difference classes with the following restrictions:

Fighters, Rangers, and Paladins: A multiclassed warrior can use all of his abilities without restriction. The warrior abilities form the base for other classes.

Clerics and Druids: Regardless of his other classes, a multiclassed priest must abide by the weapon restrictions of his mythos. He retains all his normal priest abilities.

Wizards: A multiclassed wizard can freely combine the powers of the wizard with any other class allowed, although the wearing of armor while casting spells is not permitted. Elves wearing elven chain can cast spells in armor, as magic is part of the nature of elves.

Thieves and Bards: A multiclassed thief or bard cannot use any thieving abilities other than Open Locks and Find Traps if he is wearing armor that is not normally allowed them. He must remove gauntlets to Open Locks.

dual-class

Dual-classed characters are humans who choose a change in profession. While, this allows humans a greater range of skills, restrictions apply when learning a new profession. Skills from the previous profession are unavailable until the character's current profession is one level higher than their previous profession. Additionally, while learning the new profession, hit points are not gained. In BG, the dual classing rules are different than in AD&D. Valid dual classes are those that are the same as the multiclasses listed above and no triple classes are permitted. This limitation was primarily for programming reasons.