Ever since the the original D&D made every Player Character pick a class, players have chaffed at the restrictions that imposed. Why can't I build half the characters in The Lord of the Rings in this system? Why can't the Fighter learn to throw a few spells around? Why can't the Rogue focus more on learning to fight than learning to skillmonkey? Why is the Wizard only able to do fucking everything in the right hands with the right mindset? Whose bright idea was it to make Elf and Dwarf their own classes?
As a result, uncharacteristically following public opinion and appeal, the tabletop RPG market actually responded by giving the player the ability to multiclass, gaining many of the advantages of two different classes at once. Usually, this also means sacrificing some of the advantages of both classes or suffering a penalty of some kind to compensate and maintain balance, meaning that characters who multiclass are sacrificing the raw, specialized power of their initial choice for increased versatility.
Second Edition offered two different ways to multiclass. Confusingly, one was called multiclassing, while the other was called dual-classing. Both were also class and racial locked, because fuck you, Gygax's Asperger's was married to frustrating and arbitrary restrictions, and he didn't care who knew it!
Multiclassing was limited to different races for different class combinations, as well as the stat, race, and alignment restrictions for both. Essentially, you had all the advantages and disadvantages of two different classes at once (a fighter/mage, for instance, couldn't cast spells in armor), but had to split all the XP earned up among all the classes you had levels in. A fighter/mage would split his XP gained between his two classes, a fighter/mage/thief would divide his XP into thirds, a (Dark Sun-exclusive) mage/fighter/cleric/psionicist would split his XP into quarters, etc. And since classes in 2nd Ed. all leveled at different rates this quickly turned into a clusterfuck for the player to manage. You could have a Level 15/14/17 elven fighter/mage/thief and your character "sheet" would require a binder.
Dual-classing was human-exclusive, because of course humans have to have the potential to be the best at anything - at least, until Red Steel came out, with Lupins, Rakastas and Aranea able to dual-class as well. At any point in his or her career, a human could, after gaining a level, decide to dual-class into another class he or she met the stat requirements for. Upon doing so, the character would be reset to level one in a second class to keep going from there. Once they reached the same level in the second class as they had originally had in the first, the character would regain all the abilities learned from the first class. It would never advance again, but he or she would keep on leveling the second class. Thus, the usual way of doing it was to start as a muscle class (for the early advantage) before becoming a spellcaster (for the late-game scaling). One small detail, though- if you resorted to using any of the powers of your former class, you forfeited ALL experience points gained from that adventure (because you're trying to learn a new thing rather than old shit), effectively not only stalling out your character's permanent advancement. This made it a desperation move, done only to save the character's life... and sometimes, depending on GM generosity and death-reversal availability, not even then!
Both were, as was the rule back then, unnecessarily complicated, messy, and a pain to manage; and doing it wrong meant crippling your character permanently, but they could still be very powerful if the players knew what they were doing.
A sort of prototype of 3rd edition's Multiclassing style, with a character picking up "functional levels in" secondary or even tertiary classes, did make an appeared in AD&D. Mostly it was reserved for "Creator's Pet" NPCs, like the Seven Sisters.
3rd Edition and Subsidiaries
Third Edition changed the way leveling worked, and multiclassing is much more straightforward.
Whenever a character gained a flat amount of XP, the same for every class, he could choose to gain a level in any class, provided, again, that he met any restrictions that class had. A barbarian could take a level in druid to become a rage-spellcaster. A sorcerer could take a level in cleric to gain access to some nice spells and gish potential. A ranger could take a level in rogue for skill points and special trap-disarming potential. A wizard could never take a level in any class that might dilute their world-smashing uber-powers. A fighter could take two levels in fighter before immediately reclassing into something else. The only limit was your imagination, and possibly the viability of building your bizarre chimera-character, though there was one frustrating bit of unnecessary wonkery: XP penalties. Taking anything but one of your "racial favored classes" as a class beyond the first gave your character a 5% XP penalty, and it stacked for every other class you took. A lot of DMs didn't see fit to enforce that, and Pathfinder outright removed it, but it still sucked for those who did have to deal with it.
Ironically, although it calls it "multiclassing", the idea actually has its roots in the Dual-Classing system of AD&D, with you completely progressing in one class at a time; the difference is the ability to both use the abilities of your past classes at all times and to switch level growth between your classes, instead of the first class being stuck at whatever level you switched over at.
This edition also gave rise to the idea of a dip class, one that a character would take only a few levels in for front-loaded initial benefits before reclassing out of for better-scaling ones. The archetypical example is the pre-Pathfinder fighter, which offered good hit-points and feat support for the first few levels before falling off later.
In general, this class and multi-class system was pretty fun and functional. It had a lot to offer for people trying to build their own unique characters within a sane framework, and the addition of Prestige classes helped encourage otherwise-neglected combinations. However, it had... internal problems:
- Monte Cook's insane caster fetish ensured that magic classes were just balls-out more powerful than others, while a lack of broad quality control often meant that many classes were often poorly-designed and broken, in both directions.
- Redundant class powers didn't always stack, and pure casters rarely took levels outside their first when doing so meant losing a full level's worth of spells and caster-level progression.
- Because each class was affected only by its own level and not the character's total level, a poorly-built character could end up being weaker than a single-classed character of the same level, especially when it came to caster classes.
- And, of course, the increased customization meant that it could be easily abused in the wrong hands, most infamously by Pun-Pun.
Multiclassing is still possible in Pathfinder, but the higher number of more useful class features later into a class make it less used, though combos of [insert class]/Fighter 2, Sorcerer/Paladin 2/Eldritch Knight (and/or Dragon Disciple) and Wizard 3/Rogue 3/Arcane Trickster X remain popular. It also changed up the formula a little with the archetype system, which lets players play modified versions of base classes with altered class features, and thus takes a lot of the necessity out of realizing unusual character concepts. In fact, the Advanced Class Guide is essentially a series of AD&D-style blent classes, which take half their features from two different classes! (The Brawler is a fighter/monk, the Bloodrager is a barbarian/sorcerer, etc.) While actual multiclassing has thus become more rare, the spirit of multiclassing is stronger than ever.
For those that desperately want their character to be a combination of two classes but don't want to lose out on high level abilities, Pathfinder Unchained included rules for "Variant Multiclassing". Which was basically exchanging five feats from your characters progression in exchange for some of the core mechanics of another class such as Sorcerer Bloodlines, Barbarian Rage or Cleric Domains/Channeling that typically advance at about half the normal rate without interfering with your "Primary" class progression. Sadly some of the options are stupidly imbalanced where you've got variants like the Witch that only get a familiar and non-scaling hex that prohibits you from taking the good ones, by which point you won't care any more. While losing five feats can be crippling to particular builds of character, some combinations make up for it by synergising really well with primary class mechanics (such as variant-Magus with Bloodrager; or variant-Rogue with just about anything). Other combinations such as variant-Witch or variant-Wizard? not so much... Like many parts of Pathfinder Unchained it was really just a beta test for PF2E, where all multiclassing works this way.
Quit your whining son. It was a thing.
Characters could take a feat at any level to multiclass in another class. Doing so was more akin to dipping or selecting an archetype than anything else: while their primary class would still be dominant, multiclassing allowed the player to select powers, skill training, and features from the off-class, and counting as it for the purposes of unlocking Paragon Paths and Epic Destinies. Only a bard could do it more than once, but it could still be fun under the right circumstances and enough feats.
Multiclassing in this way also granted a variant PP called Paragon Multiclassing, which essentially let the player swap out an at-will power for one from their second class and learn powers at level 11 (encounter), 12 (utility), and 20 (daily). Unfortunately, this was far less than ideal for practically anyone, as the powers you got didn't really stack up to what you could get from a proper path and you also got no special features, which was how players got both new bonuses (including training in skills or special situational benefits) and extra ways to add an extra feature to spending an Action Point.
There were also hybrid classes that were (ironically) essentially the model that Pathfinder would pick for its own "blent" classes. A hybrid picks two classes to hybridize. He or she receives only the armor proficiencies that both classes have, but gets all the weapon and implement proficiencies and trained skill choices from both classes, hit points and healing surges equal to the average of the two classes (rounded down and before adding Constitution bonuses), and their choice of traits, abilities, and powers from both of the two classes they hybridize. It's labor-intensive, but interesting, to say the least.
Ironically, this hybrid classing system is actually closer in nature to the actual multiclassing system of AD&D, as it ensures both of your "constituent" classes grow at roughly the same pace and your overall ability level is identical to the rest of the party.
Multiclassing in 5e is essentially a blend of third and fourth edition multiclassing. Whenever you level up you can choose to take a level in another class, as per third edition rules. However, multiclassing doesn't give the same proficiencies as taking the first level in a class and the earlier class will remain dominant, like in fourth edition non-hybrid classes. For example, a Fighter 1/Bard 1 will have heavy armor, martial weapons, etc, but will only have one instrument and one skill, whereas a Bard 1/Fighter 1 will have three instrument and skill proficiencies, but no heavy armor.
Of course, multiclassing has been made somewhat less prevalent by the addition of archetypes, though unlike Pathfinder archetypes these are automatic class components rather than optional features that change the class. Each class splits up into branches (most have three, and some have as many as eight, all have at least two) at a certain level that must be chosen from, specializing their abilities in some way. For example, a 3rd level fighter can become a Champion (more likely to crit, better Athletics checks, eventually gains beefy fast healing), a Battlemaster (gets battle maneuvers to trip/disarm/frighten enemies, has more benefits for fighting tactically and being a party leader) or a retooled Eldritch Knight (who can cast spells and eventually learns to do it whenever making attacks). As a result, mixing two classes' worth of abilities is much easier without true multiclassing, though, as per the Pathfinder equivalent, the spirit of playing a "mixed" character is still there.
We can summarize this fairly easily: systems with rigid level progression generally offer a way to multiclass, systems with point-buy progression generally do not. (Systems without classes like GURPS can be safely taken off the table for obvious reasons.)
In general, it's easy to see the reason why: the freeform nature of progression in these systems offers the same versatility that multiclassing normally would in others. And while in theory multiclassing lets you do anything, in practice players pick classes that offer some kind of synergy anyway, so said point-buy systems are essentially just adding guard rails to the established practice.
Warhammer 40,000 Roleplay
In the earlier games such as Dark Heresy and Rogue Trader, players are pretty much railroaded into their role at character creation. The options for multiclassing encompass "Alternate Careers" which only change out what you get at one level and then return you to your original progression after that. Though some of the later splatbooks brought in unique/sweeping careers which gave you abilities that you would never otherwise get (such as psychic powers), by and large your original focus always remains the same, thus for example Sisters of Battle will always stay Sisters of Battle.
Deathwatch (RPG) did away with this, and basically added layers of character progression, so you could take your options from multiple careers if you so chose. In fact starting characters got FOUR classes before even starting their first mission (Generic Space Marine, Space Marine of a specific Chapter, Deathwatch Specialist, and whatever role they choose) Adding in more Advanced Specializations was a simple buy in deal that just gives you more options and never restricted you from what you already had.
Black Crusade and Only War took this a bit further and practically never gives the player the ability to multiclass. Instead you choose a starting package of skills and talents at character creation and after that EVERY skill/talent is available to you, modified only by your ever shifting alignment (in Black Crusade) or allowing you to periodically change your aptitudes (in Only War). So you can change your role into whatever you can dream of using the rules.
This game never lets a player play two castes at once or to change one's class once it's locked in. One of the iconics actually has angst about this, since the Unconquered Sun made her an unparalleled assassin specifically because he knew she wouldn't enjoy it. (It's... it's not really a punishment thing so much as a "won't abuse her power thing.")
Short of weird, game-breaking shit (like diablerie or turning into an abomination), the stuff you pick at character creation is the stuff you're stuck with.
This game is a bit of an unusual situation, because your "class" system is much, much more social-based. See, instead of Classes you have Schools, which are taught either by one of the various clans and/or families in the setting, or run by some group with very specific requirements on who can join them. This is based largely on certain aspects of older Japanese society where your position is much more stratified.
As a result of this unusual social aspect to the setting, multiclassing is a much bigger issue than in games like D&D. In the first place, you have to pick up the Multiple Schools advantage each time you want to switch things up... and this is basically telling your old School that you have learned all you ever intend to learn from them, and you are quite done with them for the rest of your life. Additionally, you have to have some major allies, connections, or favors in the new place you're going to learn from to justify them teaching you this new way of life, which can mean the GM may require you to take additional advantages or disadvantages to represent these other factors.
Now, you may think this is all arbitrary. Sorry, but Advantages cost double their point value in XP after character creation, meaning you are definitely falling behind everyone else just to pick up a few new entry-level tricks. If this doesn't seem a bad thing, keep in mind that some of the Schools have some truly powerful abilities at Rank 3+. For example, many schools do not give you an extra attack until at least Rank 3 or higher, and dipping out of shugenja to pick up swordsmanship means you'll never be able to call upon the really powerful spells in the game.
There are, naturally, a few exceptions. You can always learn any of the "basic" Ronin schools that aren't attached to a specific group. This isn't so bad, to be honest: a lot of Minor Clans do not get their own schools for certain things, so the Ronin schools are essentially the only avenue some of them have to train in roles like bushi or shugenja. But mechanically, the Ronin schools are definitely inferior in a lot of ways to their Great Clan counterparts: if your Mirumoto Bushi gets kicked out of the Dragon Clan, you are truly fucked if you think Ronin Warrior or Ronin Duelist will ever make up for some of your later techniques.
Having said all of this... it's not actually a big deal. There's a lot of other tricks you can get to make up certain aspects of the game your character needs to be good at. Your Kakita Duelist can, in fact, pick up Etiquette and Courtier skills to be almost as socially adept as your Doji Courtier cousins. There is absolutely nothing stopping your shugenja from stacking up on Crab Hands and weapon skills; in fact, a lot of Kuni witchhunters do precisely this, because they have to engage in as much physical combat as their Warhammer Fantasy counterparts. And your courtier can certainly learn combat skills like Jiujitsu and Defense, the better to protect themselves against assassins. (Ninjas actually have it bad in this game: they need almost every skill imaginable for a variety of situations, because they are functionally the James Bonds of the setting, only if they fuck up nobody is going to save them from execution by the authorities.)
So the long story short here is, you can multiclass, but you shouldn't bother wasting time and XP doing it unless you have a very, very specific reason to do so. Just buy more Advantages and Skills and call it a day.
This comes the closest, letting players buy access to other specialization skill trees from outside their base class at a price hike, but even here that's more of a way to gain access to skill unlocks and unique abilities than to seamlessly blend strengths. In fact, "classes" in these games tend to mostly exist to grant price breaks and "unlocks" for particular skills and talents than to offer hard benefits!
GURPS Dungeon Fantasy
Well, GURPS is classless system, but Dungeon Fantasy provides rules to make it classy. Characters are build from 250-point templates that more or less correspond with D&D classes, and then advance them by taking those advantages and skills from their template they haven't bought before (attributes have no limitations). However they can scrounge 50 points for a Lens (package of stuff from another template; Knights and Swashbucklers have enough spare points to buy it during character creation), and after that GM has two options - either they start getting abilities from two templates right off the bat, or they have to buy all the mandatory attributes, advantages and skills for their new "class" before they can do that. Finally, "Henchmen" splatbook allows players to combine a 125-point template with 125-point lens to get a hybrid character. Or if GM wills it, players can just disregard all that and do whatever the fuck they want.
Multiclassing here is more along the lines of multiclassing in 2E or hybrid classes in 4E, in which two classes progress at the same time, though with reduced potency. For starters, multiclassing demands that you split your starting three talents (which give unique features to your classes) between both classes while your HP and Recovery Die is averaged between both classes and your class-exclusive stat boost can be assigned to any stat covered by either class. However, while you can select the highest between your classes for AC, PD, and MD, not only is the damage for every weapon attack reduced by one die size (Unless both classes are non-casting classes and aren't Monks), but you suffer any penalties associated with attacking with classes incompatible with your armor (Thus Fighter/Wizards suffer penalties for casting spells while in heavy armor) and your class ability and talent progression is held back by one step. Thus, no caster will ever gain their strongest spells when multiclassing.