If you just have one spellcasting class - say, the Wizard/Fighter I mentioned above - then you may be thinking we’re good, I just have the one class that can cast spells, I just use the rules for that class and level. We haven’t talked about what happens when you multiclass between two or more spellcasting classes. Okay, you’re thinking - that’s a bit complicated, but I have a basic understanding. How spellcasting complicates multiclassing So Barbarians who take levels in Monk don’t get to add their Dex, Con and Wisdom to their AC. Similarly, Barbarian and Monk Unarmored Defense (or any other kind of Unarmored Defense that might happen down the line if a new class with it is added) doesn’t stack, and in fact, you can’t even get another kind of Unarmored Defense if you already have it. This is also the case for Channel Divinity - if you’re a Cleric who multiclasses into Paladin, you’ll get different types of Channel Divinity, but you only get extra uses of it when you gain a level in a class that specifically states it grants an extra use of Channel Divinity. For example, if you already have Extra Attack (you’re a 5th level Barbarian who decides to multiclass into Fighter or Paladin, for example) then your Extra Attack from your Barbarian levels do not stack with Fighter or Paladin Extra Attack - if you’re a 10th level character with 5 Barbarian and 5 Fighter levels, you do not get another Extra Attack when you hit Fighter 5. When you multiclass into a new class, you get the features of that class at that level except for some special circumstances. Each class has a certain amount of proficiencies like armor types, weapon types, and in the case of classes like Bards skills and musical instruments you learn by taking a level in the class, which you can find on pages 163 and 164 of the Player’s Handbook.
Okay, so what do you get, and what don’t you get when you multiclass? Well, for starters, you don’t get all the proficiencies. So in the case of your Wizard/Fighter above, if you took three levels of Wizard before starting to take Fighter levels, and took two levels of Fighter, you’re a Wizard 3/Fighter 2, which makes you a 5th level character, and thus your Proficiency Bonus is a +3, just as it would have been for a 5th level Wizard or 5th level Fighter. Your Proficiency Bonus is always based on your character level, not your class level. Now, certain things are pretty straightforward. That 14 in Con is because it’ll help with HP while you’re trying to get ready to switch classes - a reasonably high Con is good for every single class, that’s why it’s not considered a prerequisite for any class. If your Wizard starts off with the standard array of 15, 14, 13, 12, 10 and 8, for example, and you’re planning to multiclass into Fighter later, you might want to put your 15 in Intelligence, your 14 in Constitution, and the 13 in Strength. If you’re planning to multiclass from the jump, it might be worth it to put a strong stat in something you normally wouldn’t. So, for example, it’s easier to multiclass into a class you already have a high primary attribute in - so if you wanted to take a level of Paladin on your Warlock, you probably at least have that Charisma at or above a 13, but the Strength could be harder to come by unless you did your research in advance. You can see on the chart above what the primary attributes are for each class. These stats are called your prerequisites. So if you only have a 12 Str, you can’t multiclass as a Fighter. Great! Except in order to multiclass, you have to have a 13 or higher in the primary attribute for the class you’re currently playing (Wizard, in this case) and in the class you intend to pick up (Fighter).
Let’s say you made third level as a Wizard when you decided you wanted to focus on melee combat, and when you reached fourth level, you decided to multiclass as a Fighter. For the purposes of this example, we’ll say you started as a Wizard. You start off like all characters - your first level will be in a specific class. Multiclassing is, on the surface, very simple. So let’s go over multiclassing, how it functions, and whether or not you should give it a try. Certain class combinations work extremely well together, and others really do not. So how does it work, and should you be looking at multiclassing? Well, to answer the second question first - the whole post is going to be about how it works - I’ll just say that there are significant pros and cons to multiclassing. Multiclassing in previous editions worked differently per edition - the way it worked in AD&D or in 3rd Edition is not how it works now.
Multiclassing in Dungeons and Dragons 5e can be rewarding, but it can also end up cheating you out of abilities you’d otherwise get - it mixes together aspects of various classes and does so in a way unique to this edition of the game.