Today, we released Master of None: Multiclassing Variants and Roleplay Suggestions on the DM’s Guild. I wrote the section on gestalt characters and about half of the section that provided a historical perspective on multiclassing. One thing I touched on there, but didn’t go into much detail on, is how there are primarily two very different ways in which multiclassing has been approached in D&D. To keep them clear, I’m going not going to generally refer to these approaches with terms that have been used in the rules, as these change depending on edition. Instead, I’ll call the two approaches serial multiclassing and parallel muliclassing.
Serial multiclassing is what most people think of as multiclassing: you begin in one class and you can, at some point, switch to another. This model came to prominence in third edition, and it remains the basic model for multiclassing in fifth edition. It was, in some ways, the original model for multiclassing. In the original edition of D&D, humans could advance in one class and abandon it for another. Elves could switch back and forth between advancing as a Fighting-Man and a Magic-User, but could only advance as one at a time.
In Advanced Dungeons & Dragons, these types of character builds became known as “dual-classed” character. Only humans could be dual-classed for the most part, though 1e AD&D had an exception (surprise!) for half-elves who sought to become bards. Dual-classed characters were also limited in several ways. Once you switched classes, you couldn’t go back to an old class. Moreover, until you had reached a level in your new class that equalled what you’d achieved in your first one, you suffered penalties if you used skills from your old class while advancing in your new class. If that seems overly restrictive to you, you’re not alone. Except for a few builds, dual-classed characters were generally unpopular in AD&D. It took jettisoning all the limitations on them to make them into a popular option.
Parallel multiclassing was introduced in 1975 with the first supplement to the original rules. In Greyhawk, elves, half-elves, and dwarves could advance in more than one class at a time (splitting their experience points between the classes equally). The class combinations they could choose were limited based on their race, and they could only advance to certain levels in some classes.
In AD&D, this became known as multi-classing. Again, it was only open to non-humans, the available combinations were restricted based on the character’s race, and there were class level limits also based on race. Among many players, this was more popular than dual-classing at the time, with things like elven fighter/magic-users, dwarven fighter/clerics, gnome illusionist/thieves, and half-orc cleric/assassins becoming popular archetypes.
With third edition, racial restrictions were essentially eliminated, and—with the limitations on serial multiclassing removed—there didn’t seem to be much need for a parallel option. Third edition was nothing if not completist, though, so a method of parallel multiclassing was re-introduced in Unearthed Arcana: the gestalt character. A gestalt character simply chose two classes and took the best options from each of them at each level, advancing in both simultaneously. These characters were explicitly not designed to mix well with other PCs. Instead, they were geared towards a more high-powered game.
Similarly, there really isn’t a version of parallel multiclassing in 5e. Some subclasses come close, of course. For instance, the Eldritch Knight and the Bladesinger are designed to evoke the Fighter/Wizard. In Master of None, I addressed this with a number of 5e gestalt options. Several of these were designed explicitly not to have a significant power gap with standard PC builds. If you check it out (even if you don’t buy it—you can read the entire book via the preview on the Guild), please let me know what you think.